Imaging device having a voltage supply circuit supplying potential differences between electrodes of dual imaging cells

ABSTRACT

An imaging device including a first imaging cell having a first photoelectric converter including a first electrode, a second electrode, and a first photoelectric conversion layer between the first electrode and the second electrode, and a first reset transistor one of a source and a drain of which is coupled to the first electrode; and a second imaging cell having a second photoelectric converter including a third electrode, a fourth electrode, and a second photoelectric conversion layer between the third electrode and the fourth electrode, and a second reset transistor one of a source and a drain of which is coupled to the third electrode. The imaging device further including a first voltage supply circuitry to supply a first voltage to the first reset transistor; and a second voltage supply circuitry to supply a second voltage different from the first voltage to the second reset transistor.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/876,263, filed on Jan. 22, 2018, which claims the benefit of JapaneseApplication No. 2017-019090, filed on Feb. 3, 2017, Japanese ApplicationNo. 2017-019091, filed on Feb. 3, 2017, and Japanese Application No.2017-182534, filed on Sep. 22, 2017, the entire disclosures of whichapplications are incorporated by reference herein.

BACKGROUND 1. Technical Field

The present disclosure relates to an imaging device.

2. Description of the Related Art

Image sensors utilizing photoelectric conversion are widely used. As areplacement for pinned photo diodes, a structure in which aphotoelectric conversion device is arranged above a semiconductorsubstrate has been proposed (see International Publication Nos. WO2014/024581 and WO 2014/002367, Japanese Unexamined Patent ApplicationPublication No. 2008-227092). The photoelectric conversion devicepositioned above a semiconductor substrate typically has a pair ofelectrodes, and a photoelectric conversion film interposed between theseelectrodes. In the structure in which a photoelectric conversion deviceis arranged above a semiconductor substrate, wires can be disposedbetween the semiconductor substrate and the photoelectric conversiondevice. Thus, the structure has an advantage in that the area of regionthat contributes to photoelectric conversion can be increased in eachimaging cell, as compared with the case where pinned photo diodes areused.

International Publication No. WO 2014/024581 and Japanese UnexaminedPatent Application Publication No. 2008-227092 each disclose aphotoelectric conversion device capable of changing a spectrumsensitivity by switching between bias voltages to be applied to a pairof electrodes. With the technique described in International PublicationNo. WO 2014/024581 and Japanese Unexamined Patent ApplicationPublication No. 2008-227092, a structure in which organic photoelectricconversion films with different absorption spectra are stacked isdisposed between the pair of electrodes.

International Publication No. WO 2014/024581 discloses formation of aphotoelectric conversion film between a pair of electrodes byco-evaporation of two or more types of organic semiconductor materials.Japanese Unexamined Patent Application Publication No. 2008-227092describes that one of monochrome images based on an infrared image andvisible light can be selectively obtained by switching the bias voltageto be applied across a pair of electrodes between a first bias voltageand a second bias voltage. The contents of the disclosure ofInternational Publication Nos. WO 2014/024581 and WO 2014/002367,Japanese Unexamined Patent Application Publication No. 2008-227092 areincorporated by reference herein in its entirety.

SUMMARY

It is useful when multiple image signals obtained with differentsensitivities can be collectively acquired by a single imaging device

In one general aspect, the techniques disclosed herein feature thefollowing: an imaging device including: a first imaging cell having afirst photoelectric converter including a first electrode, a secondelectrode, and a first photoelectric conversion layer between the firstelectrode and the second electrode, and a first reset transistor one ofa source and a drain of which is coupled to the first electrode. Theimaging device further including: a second imaging cell having a secondphotoelectric converter including a third electrode, a fourth electrode,and a second photoelectric conversion layer between the third electrodeand the fourth electrode, and a second reset transistor one of a sourceand a drain of which is coupled to the third electrode. The imagingdevice also including a first voltage supply circuitry configured tosupply a first voltage to the other of the source and the drain of thefirst reset transistor; and a second voltage supply circuitry configuredto supply a second voltage different from the first voltage to the otherof the source and the drain of the second reset transistor.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating an overview of theconfiguration of an imaging device according to a typical embodiment ofthe present disclosure;

FIG. 2 is a diagram illustrating an exemplary circuit configuration ofan imaging device according to a first embodiment of the presentdisclosure;

FIG. 3 selectively illustrates imaging cells adjacent in the rowdirection in a pixel array illustrated in FIG. 2;

FIG. 4 is a schematic sectional view illustrating an exemplary devicestructure of an imaging cell included in the pixel array;

FIG. 5 is a schematic sectional view illustrating an example ofconfiguration in which one microlens and color filter are shared betweenthe imaging cells adjacent to each other;

FIG. 6 is a plan view illustrating an example of the shape of pixelelectrodes as seen in the normal direction of a semiconductor substrate;

FIG. 7 is a plan view illustrating another example of the shape of pixelelectrodes as seen in the normal direction of the semiconductorsubstrate;

FIG. 8 is a plan view illustrating still another example of the shape ofpixel electrodes as seen in the normal direction of the semiconductorsubstrate;

FIG. 9 is a diagram illustrating an exemplary circuit configuration ofan imaging device according to a variation of the first embodiment;

FIG. 10 selectively illustrates imaging cells adjacent in the rowdirection in the pixel array illustrated in FIG. 9;

FIG. 11 is a diagram illustrating the circuit configuration of twoimaging cells taken from the imaging cells included in the pixel arrayof the imaging device according to a second embodiment of the presentdisclosure;

FIG. 12 is a diagram illustrating an exemplary circuit configuration ofan imaging device according to a third embodiment of the presentdisclosure;

FIG. 13 selectively illustrates imaging cells adjacent in the rowdirection in the pixel array illustrated in FIG. 12;

FIG. 14 is a diagram for explaining the operation of the imaging deviceaccording to an embodiment of the present disclosure when a rollingshutter is applied to the imaging device;

FIG. 15 is a diagram illustrating the circuit configuration of twoimaging cells taken from the imaging cells included in the pixel arrayof the imaging device according to a variation of the third embodiment;

FIG. 16 is a diagram illustrating the circuit configuration of twoimaging cells taken from the imaging cells included in the pixel arrayof the imaging device according to another variation of the thirdembodiment;

FIG. 17 is a diagram schematically illustrating an exemplaryconfiguration of an imaging device according to another Example of thepresent disclosure;

FIG. 18 is a schematic sectional view illustrating an instance of aphotoelectric converter;

FIG. 19 is a diagram illustrating an instance of a material applicableto a photoelectric conversion layer;

FIG. 20 is a schematic sectional view illustrating another example ofthe photoelectric converter;

FIG. 21 is an energy diagram in still another configuration example ofthe photoelectric converter;

FIG. 22 is a diagram illustrating the chemical formula of CZBDF;

FIG. 23 is a diagram illustrating an instance of an absorption spectrumin a photoelectric conversion layer including tin naphthalocyanine;

FIG. 24 is graph illustrating the voltage dependence of external quantumefficiency in a sample of Example 1-1;

FIG. 25 is a graph illustrating the relationship between externalquantum efficiency and applied electric field with wavelengths of 460nm, 540 nm, 680 nm, and 880 nm for the sample of Example 1-1;

FIG. 26 is a graph illustrating the voltage dependence of externalquantum efficiency in a sample of Reference Example 1;

FIG. 27 is a graph illustrating the voltage dependence of externalquantum efficiency in a sample of Example 1-2;

FIG. 28 is a graph illustrating the voltage dependence of externalquantum efficiency in a sample of Comparative Example 1;

FIG. 29 is a graph illustrating the voltage dependence of externalquantum efficiency in a sample of Example 2-1;

FIG. 30 is a graph illustrating the voltage dependence of externalquantum efficiency in a sample of Comparative Example 2-1;

FIG. 31 is an energy diagram for a sample of Example 2-2;

FIG. 32 is a graph illustrating the voltage dependence of externalquantum efficiency in a sample of Example 2-2;

FIG. 33 is an energy diagram for a sample of Comparative Example 2-2;

FIG. 34 is a graph illustrating the voltage dependence of externalquantum efficiency in a sample of Comparative Example 2-2; and

FIG. 35 is a graph illustrating a typical photocurrent characteristic ofa photoelectric conversion structure according to Embodiments of thepresent disclosure.

FIG. 36 is a diagram schematically illustrating an exemplaryconfiguration of an imaging device according to still another embodimentof the present disclosure; and

FIG. 37 is a graph for explaining the voltage dependence of externalquantum efficiency of a photoelectric conversion structure.

DETAILED DESCRIPTION Underlying Knowledge of Inventors

With the technique described in Japanese Unexamined Patent ApplicationPublication No. 2008-227092, a monochrome image based on an infraredimage and visible light is obtained. However, a photograph is taken withone of the first and the second bias voltages selectively applied, andthus moments of photographing for these images are different. In otherwords, it is not possible to collectively obtain an infrared image of asubject, and a monochrome image based on visible light at a certainmoment.

It is useful if image signals based on different wavelength ranges canbe collectively obtained.

Before Embodiments of the present disclosure is described, a summary ofthe Embodiments of the present disclosure will be described. The summaryin an aspect of the present disclosure is as follows.

[Item 1]

An imaging device including:

a first imaging cell including

-   -   a first photoelectric converter including a first pixel        electrode, a first opposite electrode, and a first photoelectric        conversion layer between the first pixel electrode and the first        opposite electrode, the first photoelectric converter generating        first signal charge by photoelectric conversion, and    -   a first charge detection circuit connected to the first pixel        electrode, the first charge detection circuit detecting the        first signal charge;

a second imaging cell including

-   -   a second photoelectric converter including a second pixel        electrode, a second opposite electrode, and a second        photoelectric conversion layer between the second pixel        electrode and the second opposite electrode, the second        photoelectric converter generating second signal charge by        photoelectric conversion, and    -   a second charge detection circuit connected to the second pixel        electrode, the second charge detection circuit detecting the        second signal charge; and

a voltage supply circuit supplying a voltage such that, in a first frameperiod, a potential difference between the first pixel electrode and thefirst opposite electrode at a start time of a charge accumulation periodof the first imaging cell is made different from a potential differencebetween the second pixel electrode and the second opposite electrode ata start time of a charge accumulation period of the second imaging cell.

[Item 2]

The imaging device according to Item 1, wherein

the first charge detection circuit includes a first reset transistorhaving a source and a drain, one of the source and the drain of thefirst reset transistor being connected to the first pixel electrode, thesecond charge detection circuit includes a second reset transistorhaving a source and a drain, one of the source and the drain of thesecond reset transistor being connected to the second pixel electrode,

in the first frame period, the voltage supply circuit supplies a firstvoltage to the other of the source and the drain of the first resettransistor in a reset period of the first imaging cell, and supplies asecond voltage different from the first voltage to the other of thesource and the drain of the second reset transistor in a reset period ofthe second imaging cell.

[Item 3]

The imaging device according to Item 1, further including:

a first inverting amplifier having a first inverting input terminal, afirst non-inverting input terminal, and a first output terminal; and

a second inverting amplifier having a second inverting input terminal, asecond non-inverting input terminal, and a second output terminal,wherein

the first charge detection circuit includes

-   -   a first reset transistor having a source and a drain, one of the        source and the drain of the first reset transistor being        connected to the first pixel electrode, the other of the source        and the drain of the first reset transistor being electrically        connected to the first output terminal, and    -   a first signal detection transistor having a gate, a source and        a drain, the gate of the first signal detection transistor being        connected to the first pixel electrode, one of the source and        the drain of the first signal detection transistor being        electrically connected to the first inverting input terminal,

the second charge detection circuit includes

-   -   a second reset transistor having a source and a drain, one of        the source and the drain of the second reset transistor being        connected to the second pixel electrode, the other of the source        and the drain of the second reset transistor being electrically        connected to the second output terminal, and    -   a second signal detection transistor having a gate, a source and        a drain, the gate of the second signal detection transistor        being connected to the second pixel electrode, one of the source        and the drain of the second signal detection transistor being        electrically connected to the second inverting input terminal,

in the first frame period, the voltage supply circuit supplies a firstvoltage to the first non-inverting input terminal in a reset period ofthe first imaging cell, and supplies a second voltage different from thefirst voltage to the second non-inverting input terminal in a resetperiod of the second imaging cell.

[Item 4]

The imaging device according to Item 1, wherein

the first charge detection circuit includes a first capacitor having afirst end and a second end, the first end of the first capacitor beingconnected to the first pixel electrode,

in the first frame period, the voltage supply circuit supplies a firstvoltage to the second end of the first capacitor in the chargeaccumulation period of the first imaging cell, and supplies a secondvoltage different from the first voltage to the second end of the firstcapacitor in a reset period of the first imaging cell.

[Item 5]

The imaging device according to Item 4, wherein

the second charge detection circuit includes a second capacitor having afirst end and a second end, the first end of the second capacitor beingconnected to the second pixel electrode,

in the first frame period, the voltage supply circuit supplies a thirdvoltage to the second end of the second capacitor in the chargeaccumulation period of the second imaging cell, and supplies a fourthvoltage different from the third voltage to the second end of the secondcapacitor in a reset period of the second imaging cell.

[Item 6]

The imaging device according to Item 1, wherein

the first charge detection circuit includes a first capacitor having afirst end and a second end, the first end of the first capacitor beingconnected to the first pixel electrode,

the second charge detection circuit includes a second capacitor having afirst end and a second end, the first end of the second capacitor beingconnected to the second pixel electrode, a capacitive value of thesecond capacitor being different from a capacitive value of the firstcapacitor,

in the first frame period, the voltage supply circuit supplies a firstvoltage to the second end of the first capacitor and the second end ofthe second capacitor in charge accumulation periods of the first imagingcell and the second imaging cell, and supplies a second voltagedifferent from the first voltage to the second end of the firstcapacitor and the second end of the second capacitor in reset periods ofthe first imaging cell and the second imaging cell.

[Item 7]

The imaging device according to any one of Items 1 to 6, wherein thefirst opposite electrode and the second opposite electrode form a singlecontinuous electrode.

[Item 8]

The imaging device according to any one of Items 1 to 6, wherein thefirst opposite electrode and the second opposite electrode areelectrically connected to each other.

[Item 9]

The imaging device according to Item 1, wherein in the first frameperiod, the voltage supply circuit supplies a first voltage to the firstopposite electrode in the charge accumulation period of the firstimaging cell, and supplies a second voltage different from the firstvoltage to the second opposite electrode in the charge accumulationperiod of the second imaging cell.

[Item 10]

The imaging device according to any one of Items 1 to 9, wherein thefirst photoelectric conversion layer and the second photoelectricconversion layer form a single continuous photoelectric conversionlayer.

[Item 11]

The imaging device according to any one of Items 1 to 10, wherein

each of the first photoelectric conversion layer and the secondphotoelectric conversion layer includes a first layer and a second layerstacked one on the other,

the first layer includes a first material,

the second layer includes a second material, and

impedance of the first layer is greater than impedance of the secondlayer.

[Item 12]

The imaging device according to any one of Items 1 to 10, wherein

each of the first photoelectric conversion layer and the secondphotoelectric conversion layer includes a first layer and a second layerstacked one on the other,

the first layer includes a first material,

the second layer includes a second material, and

an ionization potential of the first material is greater than anionization potential of the second material by 0.2 eV or more.

[Item 13]

The imaging device according to Item 11 to 12, wherein the firstmaterial and the second material are both electron-donating molecules.

[Item 14]

An imaging device having an array of multiple imaging cells eachincluding a first imaging cell and a second imaging cell, wherein

the first imaging cell has a first photoelectric converter and a firstcharge detector, a second imaging cell has a second photoelectricconverter and a second charge detector,

the first photoelectric converter includes

-   -   a first electrode, a second electrode, and a first photoelectric        conversion structure positioned between the first electrode and        the second electrode,

the first charge detector includes a first transistor connected to thefirst electrode,

the second photoelectric converter includes a third electrode, a fourthelectrode, and a second photoelectric conversion structure positionedbetween the third electrode and the fourth electrode,

the second charge detector includes a second transistor connected to thethird electrode,

each of the first photoelectric conversion structure and the secondphotoelectric conversion structure includes at least part of amultilayer structure having a first photoelectric conversion layer and asecond photoelectric conversion layer, and

a potential difference applied across the first electrode and the secondelectrode at the time of start of a charge accumulation period isdifferent from a potential difference applied across the third electrodeand the fourth electrode at the time of start of the charge accumulationperiod.

With the configuration of item 14, imaging cells with different spectralsensitivity characteristic can be mixed in the pixel array of imagingcells.

[Item 15]

The imaging device according to Item 14, wherein

the first electrode is connected to a gate of the first transistor ofthe first charge detector, and

the third electrode is connected to a gate of the second transistor ofthe second charge detector.

With the configuration of item 15, non-destructive read of a signalcharge is possible.

[Item 16]

The imaging device according to Item 2, wherein

the first charge detector includes a first reset circuit having a thirdtransistor having one of a source and a drain connected to the firstelectrode,

the second charge detector includes a second reset circuit having afourth transistor having one of a source and a drain connected to thethird electrode, and

a fourth electrode of the second photoelectric converter, and a secondelectrode of the first photoelectric converter have the same potential.

[Item 17]

The imaging device according to Item 16, further including:

a first voltage line that is connected to the other of the source andthe drain of the third transistor, and supplies a first voltage to theother of the source and the drain of the third transistor; and

a second voltage line that is connected to the other of the source andthe drain of the fourth transistor, and supplies a second voltagedifferent from the first voltage in absolute value to the other of thesource and the drain of the fourth transistor.

With the configuration of item 17, the first electrode of the firstimaging cell and the third electrode of the second imaging cell can bereset to different potentials, and the bias voltage applied to thephotoelectric conversion structure at the start of the chargeaccumulation period can be made different between the first imaging celland the second imaging cell.

[Item 18]

The imaging device according to Item 16, wherein

the first reset circuit includes a first feedback circuit that performsnegative feedback of the electrical signals generated in the firstphotoelectric converter,

the second reset circuit includes a second feedback circuit thatperforms negative feedback of the electrical signals generated in thesecond photoelectric converter,

the first feedback circuit includes a first inverting amplifier,

the second feedback circuit includes a second inverting amplifier,

the inverting input terminal of the first inverting amplifier iselectrically connected to one of the source and the drain of the firsttransistor, and

the inverting input terminal of the second inverting amplifier iselectrically connected to one of the source and the drain of the secondtransistor.

With the configuration of item 18, the effect of random noise can becanceled by formation of a feedback loop.

[Item 19]

The imaging device according to Item 18, further including:

a first voltage line that is connected to the non-inverting inputterminal of the first inverting amplifier, and applies a first voltageto the non-inverting input terminal of the first inverting amplifier;and

a second voltage line that is connected to the non-inverting inputterminal of the second inverting amplifier, and applies a second voltagedifferent from the first voltage in absolute value to the non-invertinginput terminal of the first inverting amplifier.

With the configuration of item 19, the first electrode of the firstimaging cell and the third electrode of the second imaging cell can bereset to different potentials, and the bias voltage applied to thephotoelectric conversion structure at the start of the chargeaccumulation period can be made different between the first imaging celland the second imaging cell.

[Item 20]

The imaging device according to Item 16 or 18, further including a firstvoltage line, wherein

the first charge detector a first capacitor connected between the firstelectrode and the first voltage line, and

the first voltage line supplies a first voltage to the first capacitor.

With the configuration of item 20, it is possible to temporarilyincrease the potential of the first electrode of the first imaging cellin the charge accumulation period by the electrical coupling between thefirst voltage line and the charge accumulation node via the firstcapacitor.

[Item 21]

The imaging device according to Item 20, wherein

the absolute value of the first voltage is different between the chargeaccumulation period included a frame period, and the period other thanthe charge accumulation period in the frame period.

With the configuration of item 21, the bias voltage applied to the firstphotoelectric conversion structure in the charge accumulation period canbe selectively changed, and the bias voltage applied to thephotoelectric conversion structure at the start of the chargeaccumulation period can be made different between the first imaging celland the second imaging cell.

[Item 22]

The imaging device according to Item 21, wherein

the second charge detector includes a second capacitor connected betweenthe third electrode and the first voltage line, and

the capacitive value of the second capacitor is different from thecapacitive value of the first capacitor.

With the configuration of item 22, the potential difference between thefirst electrode and the second electrode, and the potential differencebetween the third electrode and the fourth electrode can be madedifferent while the voltage applied to the first signal line is used incommon.

[Item 23]

The imaging device according to any one of Items 14 to 22, wherein

the first imaging cell and the second imaging cell are arranged adjacentto each other in the array, and

the imaging device further includes a subtraction circuit that outputsthe difference between a first image signal outputted from the firstcharge detector and a second image signal outputted from the secondcharge detector.

With the configuration of item 23, an image based on infrared light canbe formed based on the difference between the level of an output signalof the first imaging cell and the level of an output signal of thesecond imaging cell.

[Item 24]

The imaging device according to any one of Items 14 to 23, wherein

the second electrode of the first photoelectric converter and the fourthelectrode of the second photoelectric converter form a single continuouselectrode.

With the configuration of item 24, it is possible to collectively applya predetermined voltage to the second electrode of the first imagingcell and the fourth electrode of the second imaging cell.

[Item 25]

The imaging device according to Items 14 or 15, wherein

the first imaging cell has a first voltage line connected to the secondelectrode,

the second imaging cell has a second voltage line connected to thefourth electrode,

the second electrode and the fourth electrode are electricallyseparated, the first voltage line supplies a first voltage to the secondelectrode at least in the charge accumulation, and

the second voltage line supplies a second voltage different from thefirst voltage in absolute value to the fourth electrode at least in thecharge accumulation.

With the configuration of item 25, the potentials of the secondelectrode of the first imaging cell and the fourth electrode of thesecond imaging cell can be made different, for instance in the chargeaccumulation, and the bias voltage applied to the photoelectricconversion structure at the start of the charge accumulation period canbe made different between the first imaging cell and the second imagingcell.

[Item 26]

The imaging device according to any one of Items 14 to 25, wherein

the first photoelectric conversion layer and the second photoelectricconversion layer include a first material and a second material,respectively, and the impedance of the first photoelectric conversionlayer is greater than the impedance of the second photoelectricconversion layer.

With the configuration of item 26, the spectral sensitivitycharacteristic of the photoelectric conversion structure can be changedby changing the potential difference applied across the first electrodeand the second electrode.

[Item 27]

The imaging device according to any one of Items 14 to 25, wherein

the first photoelectric conversion layer and the second photoelectricconversion layer include a first material and a second material,respectively, and

the ionization potential of the first material is greater than theionization potential of the second material by 0.2 eV or more.

With the configuration of item 27, the spectral sensitivitycharacteristic of the photoelectric conversion structure can be changedby changing the potential difference applied across the first electrodeand the second electrode.

[Item 28]

The imaging device according to Items 26 or 27, wherein

the first material and the second material are electron-donatingmolecules.

[Item 29]

An imaging device having an array of multiple imaging cells eachincluding a first imaging cell and a second imaging cell, wherein

the first imaging cell has a first photoelectric converter and a firstcharge detector,

the second imaging cell has a second photoelectric converter and asecond charge detector,

the first photoelectric converter includes

-   -   a first electrode, a second electrode, and a first photoelectric        conversion structure positioned between the first electrode and        the second electrode,

the first charge detector includes

-   -   a first reset circuit that is connected to the first electrode,        and resets the potential of the first electrode to a first        potential,

the second photoelectric converter includes

-   -   a third electrode, a fourth electrode, and a second        photoelectric conversion structure where positioned between the        third electrode and the fourth electrode,

the second charge detector includes

-   -   a second reset circuit that is connected to the third electrode,        and resets the potential of the third electrode to a second        potential different from the first potential,

each of the first photoelectric conversion structure and the secondphotoelectric conversion structure includes at least part of amultilayer structure having a first photoelectric conversion layer and asecond photoelectric conversion layer, and

the potential of the fourth electrode of the second photoelectricconverter in the charge accumulation is equal to the potential of thesecond electrode of the first photoelectric converter in the chargeaccumulation.

With the configuration of item 29, the first electrode of the firstimaging cell and the third electrode of the second imaging cell can bereset to different potentials, and the bias voltage applied to thephotoelectric conversion structure at the start of the chargeaccumulation period can be made different between the first imaging celland the second imaging cell.

[Item 30]

An imaging device having an array of multiple imaging cells eachincluding a first imaging cell and a second imaging cell, wherein

the imaging device includes a first voltage line to which a firstvoltage is applied,

the first imaging cell has a first photoelectric converter and a firstcharge detector,

the second imaging cell has a second photoelectric converter and asecond charge detector,

the first photoelectric converter includes

-   -   a first electrode, a second electrode, and a first photoelectric        conversion structure positioned between the first electrode and        the second electrode,

the first charge detector includes

-   -   a first reset circuit that is connected to the first electrode,        and resets the potential of the first electrode to a first        potential, and a first capacitor connected between the first        electrode and the first voltage line,

the second photoelectric converter includes

-   -   a third electrode, a fourth electrode, and a second        photoelectric conversion structure where positioned between the        third electrode and the fourth electrode,

the second charge detector includes

-   -   a second reset circuit that is connected to the third electrode,        and resets the potential of the third electrode to the first        potential,

each of the first photoelectric conversion structure and the secondphotoelectric conversion structure includes at least part of amultilayer structure having a first photoelectric conversion layer and asecond photoelectric conversion layer, and

the potential of the fourth electrode of the second photoelectricconverter in the charge accumulation is equal to the potential of thesecond electrode of the first photoelectric converter in the chargeaccumulation, and

the first voltage is higher in the charge accumulation period includedin a frame period than in the period other than the charge accumulationperiod in the frame period.

With the configuration of item 30, the bias voltage applied to the firstphotoelectric conversion structure in the charge accumulation period canbe selectively changed, and the bias voltage applied to thephotoelectric conversion structure at the start of the chargeaccumulation period can be made different between the first imaging celland the second imaging cell.

[Item 31]

An imaging device having an array of multiple imaging cells eachincluding a first imaging cell and a second imaging cell, wherein

the first imaging cell has a first photoelectric converter and a firstcharge detector,

the second imaging cell has a second photoelectric converter and asecond charge detector,

the first photoelectric converter includes

-   -   a first electrode, a second electrode, a first photoelectric        conversion structure positioned between the first electrode and        the second electrode, and a first voltage line connected to the        second electrode,

the first charge detector is connected to the first electrode,

the second photoelectric converter includes

-   -   a third electrode, a fourth electrode electrically separated        from the second electrode, a second photoelectric conversion        structure positioned between the third electrode and the fourth        electrode, and a second voltage line connected to the fourth        electrode,

the second charge detector is connected to the third electrode,

each of the first photoelectric conversion structure and the secondphotoelectric conversion structure includes at least part of amultilayer structure having a first photoelectric conversion layer and asecond photoelectric conversion layer,

the first voltage line supplies a first voltage to the second electrodeat least in the charge accumulation period, and

the second voltage line supplies a second voltage different from thefirst voltage in absolute value to the fourth electrode at least in thecharge accumulation period.

With the configuration of item 31, the potentials of the secondelectrode of the first imaging cell and the fourth electrode of thesecond imaging cell can be made different, for instance in the chargeaccumulation, and the bias voltage applied to the photoelectricconversion structure at the start of the charge accumulation period canbe made different between the first imaging cell and the second imagingcell.

Hereinafter, Embodiments of the present disclosure will be described indetail. It is to be noted that each of the Embodiments described belowrepresents a comprehensive or specific example. The numerical values,shapes, materials, components, arrangement and connection topologies ofthe components, steps, the order of the steps which are presented in thefollowing Embodiments are examples, and not intended to limit thepresent disclosure. Various aspects described in the present descriptionmay be combined as long as no contradiction occurs. Any component whichis included in the components of the following Embodiments and is notrecited in the independent claim providing the most generic concept willbe described as an arbitrary component. In the following description,components having substantially the same function are labeled with acommon reference symbol, and a description may be omitted.

FIG. 1 illustrates an overview of the configuration of an imaging deviceaccording to a typical embodiment of the present disclosure. Asschematically illustrated in FIG. 1, an imaging device 100 according toan embodiment of the present disclosure includes a semiconductorsubstrate 50, an interlayer insulation layer 52 that covers thesemiconductor substrate 50, and a photoelectric converter PC supportedby the semiconductor substrate 50 and the interlayer insulation layer52. The imaging device 100 has multiple imaging cells 10, and FIG. 1illustrates a schematic sectional view of two imaging cells 10 x and 10y among the multiple imaging cells 10. The imaging device 100 includesan imaging region which has a repetitive structure in which a unitformed by a pair of two imaging cells 10 x and 10 y is repeated.

As schematically illustrated in FIG. 1, the photoelectric converter PChas pixel electrodes 61 x and 61 y, an opposite electrode 62, and aphotoelectric conversion structure 64. The opposite electrode 62 istypically a transparent electrode, and is positioned more distant fromthe semiconductor substrate 50 than from the pixel electrodes 61 x and61 y. The photoelectric conversion structure 64 is positioned betweenthe pixel electrodes 61 x and 61 y, and the opposite electrode 62, andincludes a multilayer structure having the first photoelectricconversion layer 64 a and the second photoelectric conversion layer 64b.

The imaging cells 10 x and 10 y have photoelectric converters PCx andPCy, respectively. In this example, the photoelectric conversionstructure 64 and the opposite electrode 62 are shared between theimaging cells 10 x and 10 y. Therefore, each of the photoelectricconverters PCx and PCy is part of the photoelectric converter PC. Thephotoelectric converter PCx includes an opposite electrode 62 x, aphotoelectric conversion structure 64 x, and a pixel electrode 61 x. Theopposite electrode 62 x and the photoelectric conversion structure 64 xare part of the opposite electrodes 62 and part of the photoelectricconversion structure 64, respectively. Similarly, the photoelectricconverter PCy includes an opposite electrode 62 y which is part of theopposite electrodes 62, a photoelectric conversion structure 64 y whichis part of the photoelectric conversion structures 64, and a pixelelectrode 61 y.

The imaging cell 10 x has a charge detector CDx which includes, in part,an impurity region 50 ax formed in the semiconductor substrate 50. Theimaging cell 10 y has a charge detector CDy which includes, in part, animpurity region 50 ay formed in the semiconductor substrate 50.Hereinafter, the charge detector is also called a charge detectioncircuit. As schematically illustrated in FIG. 1, the pixel electrode 61x of photoelectric converter PCx is connected to the impurity region 50ax via a connector 54 x disposed in the interlayer insulation layer 52.Similarly, the pixel electrode 61 y of photoelectric converter PCy isconnected to the impurity region 50 ay via a connector 54 y disposed inthe interlayer insulation layer 52.

The photoelectric conversion structure 64 is irradiated with light, andgenerates charge pairs therein. The charges generated in a portionbetween the opposite electrode 62 and the pixel electrodes 61 x in thephotoelectric conversion structure 64 are generally collected as signalcharges by the pixel electrode 61 x by applying an appropriate potentialdifference across the pixel electrode 61 x and the opposite electrodes62 x. The signal charges collected by the pixel electrode 61 x aretemporarily held in a charge accumulation region which includes, inpart, the pixel electrode 61 x, the connector 54 x, and the impurityregion 50 ax, and are detected by the charge detector CDx. Similarly,the charges generated in a portion between the opposite electrode 62 andthe pixel electrodes 61 y in the photoelectric conversion structure 64are generally collected as signal charges by the pixel electrode 61 y byapplying an appropriate potential difference across the pixel electrode61 y and the opposite electrodes 62 y, then the charges are detected bythe charge detector CDy. Therefore, each of the imaging cells 10 x and10 y can be defined as the unit structure that has a pixel electrodeconnected to the charge detector via the connector.

In photographing, as schematically illustrated in FIG. 1, a first biasvoltage V1 is applied to the photoelectric conversion structure 64 x,and a second bias voltage V2 is applied to the photoelectric conversionstructure 64 y. Here, the first bias voltage V1 and second bias voltageV2 are different in absolute value (|V1| ≠|V2|). In an embodiment of thepresent disclosure, at the start of the charge accumulation periodincluded in a frame period, the potential difference between the pixelelectrode and the opposite electrode of the first imaging cell (betweenthe pixel electrode 61 x and the opposite electrode 62 x in thisexample) is different from the potential difference between the pixelelectrode and the opposite electrode of the second imaging cell (betweenthe pixel electrode 61 y and the opposite electrode 62 y in thisexample).

In the photoelectric converter PC, the photoelectric conversionstructure 64 includes a multilayer structure that has the firstphotoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b. Also, the voltage applied across a pair ofelectrodes by which the photoelectric conversion structure 64 isinterposed is switched between multiple voltages having differentabsolute values. As described in detail later, the spectral sensitivitycharacteristic is changed by the switching between voltages. Thus, in acharge accumulation period, for instance, the first imaging cell hassensitivity in the wavelength ranges of visible light and infraredlight, and the second imaging cell selectively has sensitivity in thewavelength range of visible light. In this case, the charge detector(the charge detector CDx in this example) of the first imaging celloutputs an image signal based on the visible light and infrared light,and the charge detector (the charge detector CDy in this example) of thesecond imaging cell outputs an image signal based on visible light.

Like this, according to an embodiment of the present disclosure, a setincluding at least two imaging cells with different spectral sensitivitycharacteristics can be mixed in the array of imaging cells. In otherwords, it is possible to collectively obtain image signals based onlight with different wavelength ranges. According to an embodiment ofthe present disclosure, for instance, it is possible to form an imagebased on visible light and infrared light from an image signal obtainedby one of the first and second imaging cells, and to form an image basedon visible light from an image signal obtained by the other of the firstand second imaging cells. Furthermore, it is also possible to form animage based on infrared light by calculating the difference between thelevels of output signals of the first and second imaging cells. Each ofthese images is formed from an image signal based on the amount ofsignal charges accumulated in the same charge accumulation period, andthus synchronization is guaranteed. Therefore, even when a high-speedmoving object is photographed, blur does not occur in the image based onthe difference. This is different from the case where the spectralsensitivity characteristic of the imaging cell is changed between twoframes, and multiple image signals based on light with differentwavelength ranges are successively obtained.

First Embodiment

FIG. 2 illustrates an exemplary circuit configuration of an imagingdevice according to a first embodiment of the present disclosure. Theimaging device 100A illustrated in FIG. 2 has a pixel array PA includingmultiple imaging cells arranged two-dimensionally. The pixel array PAhas multiple imaging cells each including at least one pair of imagingcells 10Ax and 10Ay.

FIG. 2 schematically illustrates an example in which imaging cells arearranged in a matrix. A pixel block including four imaging cellsarranged in two rows by two columns among the multiple imaging cells isselectively illustrated. The pixel block illustrated in FIG. 2 has twoimaging cells 10Ax and two imaging cells 10Ay. In this example, theimaging cell 10Ax and 10 y are adjacent to each other in the rowdirection, and adjacent to each other in the column direction too. Inthe present description, the “row direction” indicates the direction inwhich the rows of the multiple imaging cells extend, and the “columndirection” indicates the direction in which the columns of the multipleimaging cells extend. In the example of FIG. 2, the “row direction” isthe horizontal direction on the paper, and the “column direction” is thevertical direction on the paper. Needless to say, the number andarrangement of the imaging cells 10Ax and 10Ay are not limited to theexample illustrated in FIG. 2.

The imaging cell 10Ax has the photoelectric converter PCx and the chargedetector CDx. The charge detector CDx detects a signal charge collectedby the pixel electrode 61 x (see FIG. 1) of the photoelectric converterPCx. In this example, the charge detector CDx has a signal detectiontransistor 21, a reset transistor 22, and an address transistor 23. Thesignal detection transistor 21, the reset transistor 22, and the addresstransistor 23, are typically field effect transistors (FET). Hereinafteran example, in which N-channel MOS is used for the signal detectiontransistor 21, the reset transistor 22, and the address transistor 23,will be described.

The imaging cell 10Ay has the photoelectric converter PCy and the chargedetector CDy. The charge detector CDy detects a signal charge collectedby the pixel electrode 61 y (see FIG. 1) of the photoelectric converterPCy. Similarly to the charge detector CDx of the imaging cell 10Ax, thecharge detector CDy has the signal detection transistor 21, the resettransistor 22, and the address transistor 23.

The imaging device 100A has a vertical signal line 34 provided for eachcolumn of the multiple imaging cells, and a power source wire 36 forsupplying power supply voltage AVDD to the imaging cells 10Ax and 10Ay.The drain of the above-mentioned signal detection transistor 21 isconnected to the power source wire 36, and the source of the signaldetection transistor 21 is connected to a corresponding one of multiplevertical signal lines 34 via the address transistor 23.

As schematically illustrated in FIG. 2, the gate of the signal detectiontransistor 21 of the imaging cell 10Ax is connected to the photoelectricconverter PCx. The gate of the signal detection transistor 21 is coupledto the pixel electrode 61 x of the photoelectric converter PCx. Thesignal charges generated by the photoelectric converter PCx istemporarily accumulated in the charge accumulation region in the imagingcell 10 x. The charge accumulation region of the imaging cell 10 xincludes a charge accumulation node FDx between the gate of the signaldetection transistor 21 and the photoelectric converter PCx. The chargeaccumulation node is also called a floating diffusion node. The signaldetection transistor 21 of the charge detector CDx outputs a signalgenerated by the photoelectric converter PCx with the power source wire36 serving as a source follower power supply. In other words, a voltageaccording to the signal charges accumulated in the charge accumulationregion is read to the vertical signal line 34. It is to be noted thatthe connection terminal of the signal detection transistor 21 connectedto the photoelectric converter PCx is not limited to the gate electrode.For instance, the connection terminal may be the source or the drainaccording to the circuit configuration of a charge detection circuit.

The circuit configuration of the imaging cell 10Ay is substantially thesame as the circuit configuration of the imaging cell 10Ax. The gate ofthe signal detection transistor 21 of the imaging cell 10Ay is connectedto the pixel electrode 61 y of the photoelectric converter PCy. Thesignal charges generated by the photoelectric converter PCy aretemporarily accumulated in the charge accumulation region that includesa charge accumulation node FDy between the gate of the signal detectiontransistor 21 and the photoelectric converter PCy. The signal detectiontransistor 21 of the charge detector CDy outputs a signal generated bythe photoelectric converter PCy.

The vertical signal line 34 is connected to a column signal processingcircuit 44 which is also called a row signal accumulation circuit. Thecolumn signal processing circuit 44 performs noise suppression signalprocessing represented by Correlated Double Sampling, andanalog-to-digital conversion. The column signal processing circuit 44 isprovided for each row of the imaging cells 10A in the pixel array PA.These column signal processing circuits 44 are connected to a horizontalsignal read circuit 46 which is also called a column scanning circuit.The horizontal signal read circuit 46 successively reads signals frommultiple column signal processing circuits 44 to the horizontal commonsignal line 45.

The imaging device 100A further has a first voltage supply circuit 41and a second voltage supply circuit 42. In the configuration illustratedin FIG. 2, the first voltage supply circuit 41 is connected to a firstvoltage line 31 and a second voltage line 32. The first voltage line 31has connection with each imaging cell 10Ax, and the second voltage line32 has connection with each imaging cell 10Ay. Therefore, the firstvoltage supply circuit 41 can supply voltage a predetermined V_(RST) 1to each imaging cell 10Ax via the first voltage line 31, and can supplyvoltage a predetermined V_(RST) 2 to each imaging cell 10Ay via thesecond voltage line 31. Here, the voltage V_(RST) 2 supplied to eachimaging cell 10Ay has an absolute value different from the absolutevalue of the voltage V_(RST) 1 supplied to each imaging cell 10Ax. Thefirst voltage line 31 is not limited to a single wire. The first voltageline 31 may be a structure that is electrically coupled or connected toan imaging cell to which a voltage is to be supplied, for instance, maybe a grid-like structure. This also applies to the second voltage line32 which may be a structure that is electrically coupled or connected toan imaging cell to which a voltage is to be supplied. The second voltageline 32 is also not limited to a single wire, and may be a grid-likestructure. This also applies to other voltage lines, signal lines,control lines, and wires in the present disclosure. In the presentdescription, the term “line” or “wire” used as the name of a member isonly for the sake of convenience of description, and it is not intendedto limit a specific structure, such as a voltage line, a signal line, acontrol line, and a wire, to a single linear conductor.

In this example, the voltage supply circuit 41 includes a voltage supplycircuit 41 a connected to the first voltage line 31, and a voltagesupply circuit 41 b connected to the second voltage line 32. At the timeof operation of the imaging device 100A, the voltage supply circuit 41 aapplies the voltage V_(RST) 1 to the first voltage line 31, and thevoltage supply circuit 41 b applies the voltage V_(RST) 2 to the secondvoltage line 32. Each of the voltage supply circuits 41 a and 41 b maybe an independent separate voltage supply circuit, or may be part of asingle voltage supply circuit.

In the configuration illustrated to FIG. 2, the first voltage line 31 isconnected to the source of the reset transistor 22 of the chargedetector CDx. The drain of the reset transistor 22 of the chargedetector CDx is connected to the charge accumulation node FDx. That is,the first voltage line 31 supplies a voltage V_(RST) 1 to be appliedfrom the first voltage supply circuit 41 to the charge detector CDx ofeach imaging cell 10Ax, the voltage V_(RST) 1 serving as a reset voltagefor resetting the potential of the charge accumulation node FDx. Thesecond voltage line 32 is connected to the source of the resettransistor 22 of the charge detector CDy. The drain of the resettransistor 22 of the charge detector CDy is connected to the chargeaccumulation node FDy. That is, the second voltage line 32 supplies avoltage V_(RST) 2 to be applied from the first voltage supply circuit 41to the charge detector CDy of each imaging cell 10Ay, the voltageV_(RST) 2 serving as a reset voltage for resetting the potential of thecharge accumulation node FDy.

In the configuration illustrated to FIG. 2, the second voltage supplycircuit 42 is connected to an accumulation control line 35 that hasconnection with the opposite electrode 62 (see FIG. 1) of thephotoelectric converter PC. That is, in this example, the voltage supplycircuit 42 supplies a common predetermined voltage V_(OPP) to theopposite electrode 62 x of the photoelectric converter PCx of eachimaging cell 10Ax and the opposite electrode 62 y of the photoelectricconverter PCy of each imaging cell 10Ay via the accumulation controlline 35. Hereinafter, for the sake of convenience of description, thevoltage supplied by the voltage supply circuit 42 may be called“opposite electrode voltage”. The opposite electrode voltage V_(OPP) isfixed to a certain voltage at the time of operation of the imagingdevice 100A.

The imaging device 100A has a reset signal line 38 and an address signalline 39 which are provided for each row of the imaging cell 10A. Thereset signal line 38 and the address signal line 39 are connected to avertical scanning circuit 48 which is also called a row scanningcircuit. As illustrated, the reset signal line 38 is connected to thegate of the reset transistor 22 of the imaging cells (here, the imagingcells 10Ax and 10Ay) belonging to the same row. The vertical scanningcircuit 48 controls the potential of the reset signal line 38 and turnson the reset transistor 22, thereby making it possible to collectivelyreset the potentials of the charge accumulation nodes (here, the chargeaccumulation nodes FDx and FDy) of the imaging cells belonging to thesame row. In contrast, the address signal line 39 is connected to thegate of the address transistor 23 of each of the multiple imaging cells(here, the imaging cells 10Ax and 10Ay) belonging to the same row. Thevertical scanning circuit 48 can select the multiple imaging cellsbelonging to the same row on a row-by-row basis by controlling thepotential of the address signal line 39. The multiple imaging cellsbelonging to the same row are selected on a row-by-row basis, thus theoutputs of the signal detection transistors 21 of the imaging cellsbelonging to the same row can be collectively read to a correspondingvertical signal line 34.

FIG. 3 selectively illustrates the imaging cells 10Ax and 10Ay adjacentin the row direction in the pixel array PA illustrated in FIG. 2. Asdescribed with reference to FIG. 2, the accumulation control line 35 isconnected in common to the opposite electrode 62 x of the imaging cell10Ax and the opposite electrode 62 y of imaging cell 10Ay. Thus, in thisexample, at the time of operation of the imaging device 100A, a commonopposite electrode voltage V_(OPP) is applied from the voltage supplycircuit 42 to the opposite electrodes 62 x and 62 y via the accumulationcontrol line 35. Therefore, the opposite electrodes 62 x and 62 y havethe same potential.

The voltage supply circuit 42 applies the opposite electrode voltageV_(OPP) to the opposite electrodes 62 x and 62 y so that the potentialof the opposite electrodes 62 x and 62 y is higher than the potential ofthe pixel electrodes 61 x and 61 y, for instance. For instance, when thepotential of the opposite electrode 62 x is made higher than thepotential of the pixel electrode 61 x, the positive charges in thepositive and negative charges generated in the photoelectric conversionstructure 64 x by photoelectric conversion can be collected by the pixelelectrode 61 x. Similarly, when the potential of the opposite electrode62 y is made higher than the potential of the pixel electrode 61 y, thepositive charges in the positive and negative charges generated in thephotoelectric conversion structure 64 y by photoelectric conversion canbe collected by the pixel electrode 61 y. Hereinafter, a case will beillustrated where a positive hole is utilized as a signal charge.Needless to say, it is also possible to utilize an electron as a signalcharge. When an electron is utilized as signal charge, it is onlynecessary to perform control to make the potential of the oppositeelectrodes 62 x and 62 y lower than the potential of the pixelelectrodes 61 x and 61 y.

As illustrated in FIG. 3, the charge detector CDx of the imaging cell10Ax has a reset circuit RSx1 that includes the reset transistor 22having a source connected to the first voltage line 31. Similarly, thecharge detector CDy of the imaging cell 10Ay has a reset circuit RSy1that includes the reset transistor 22 having a source connected to thesecond voltage line 32. When the reset transistor 22 of the resetcircuit RSx1 is turned on, the potential of the pixel electrode 61 x isreset by a potential corresponding to the reset voltage V_(RST) 1applied to the first voltage line 31. Similarly, when the resettransistor 22 of the reset circuit RSy1 is turned on, the potential ofthe pixel electrode 61 y is reset by a potential corresponding to thereset voltage V_(RST) 2 applied to the second voltage line 32.

The potential of the pixel electrode 61 x and the potential of the pixelelectrode 61 y at the start of a period of accumulation of signalcharges can be made different by supplying the reset voltages V_(RST) 1and V_(RST) 2 with different absolute values to the reset circuit RSx1and the reset circuit RSy1, respectively. Hereinafter, a period ofaccumulation of signal charges is simply called a “charge accumulationperiod”. After the reset of the potential of the pixel electrode 61 x,the potential difference Φx between the opposite electrode 62 x and thepixel electrode 61 x immediately after turning off the reset transistor22 of the charge detector CDx is expressed by V_(OPP)−V_(RST) 1.Similarly, after the reset of the potential of the pixel electrode 61 y,the potential difference Φy between the opposite electrode 62 y and thepixel electrode 61 y immediately after turning off the reset transistor22 of the charge detector CDy is expressed by V_(OPP)−V_(RST) 2. Here,the opposite electrode voltage V_(OPP) is nearly constant over thecharge accumulation period, and V_(RST) 1≠V_(RST) 2. Therefore, at thestart of the charge accumulation period, in other words, at a time pointimmediately after the reset of the potential of the pixel electrode andbefore start of accumulation of charges in the charge accumulationregion, the relationship Φx≠Φy holds. In other words, the bias voltageapplied to the photoelectric conversion structure 64 at the start ofaccumulation of signal charges can be made different between the imagingcell 10Ax and the imaging cell 10Ay. When the imaging cell 10Ax and theimaging cell 10Ay are arranged close to each other, for instance,adjacent to each other, the quantities of light incident to the imagingcell 10Ax and the imaging cell 10Ay are approximately equal. Therefore,in this case, when the charge accumulation region is in common withthese, it can be said that the difference between the amounts ofelectrical change in each charge accumulation region is small, and therelationship of Φx≠Φy holds over the charge accumulation period. What isnoteworthy here is that conditions are intentionally made such that therelationship of Φx≠Φy holds between the imaging cell 10Ax and imagingcell 10Ay at the start of accumulation of signal charges. When thephotoelectric conversion structure is not irradiated with light at allduring the charge accumulation, the potential difference Φx between theopposite electrode 62 x and the pixel electrode 61 x at the start of thecharge accumulation period is maintained until the end of the chargeaccumulation period. Similarly, the potential difference Φy between theopposite electrode 62 y and the pixel electrode 61 y at the start of thecharge accumulation period is maintained until the end of the chargeaccumulation period. This also applies to other embodiments describedbelow.

In this embodiment, the photoelectric conversion structure 64 of thephotoelectric converter PC includes a multilayer structure having thefirst photoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b. The first photoelectric conversion layer 64 a andthe second photoelectric conversion layer 64 b include the firstmaterial and the second material, respectively. Electron-donatingmolecules are typically selected as the first material and the secondmaterial.

Appropriate materials are used as the first material and the secondmaterial according to a wavelength range to be detected. Differentspectral sensitivity characteristics can be achieved between thephotoelectric converter PCx and the photoelectric converter PCy, forinstance, by setting the impedance of the first photoelectric conversionlayer 64 a to be greater than the impedance of the second photoelectricconversion layer 64 b. Let D1 and Z1 be respectively the thickness andthe impedance of the first photoelectric conversion layer 64 a, and D2and Z2 be respectively the thickness and the impedance of the secondphotoelectric conversion layer 64 b, then magnitude E1 of the electricfield applied to the first photoelectric conversion layer 64 a andmagnitude E2 of the electric field applied to the second photoelectricconversion layer 64 b are given by the following Expressions (1) and(2).E1=(Z1/(Z1+Z2))(Φ/D1)  (1)E2=(Z2/(Z1+Z2))(Φ/D2)  (2)Here, Φ represents the potential difference applied across the pixelelectrode and the opposite electrode.

As described above, excitons generated by incident light to thephotoelectric conversion structure 64 are efficiently separated intocharges by the electric field caused by the potential difference appliedacross the pixel electrode and the opposite electrode, and for instance,positive charges are collected as signal charges by the pixel electrode.Here, let E_(TH) be the magnitude of electric field necessary forseparation of charges and movement of charges in the photoelectricconversion structure. For instance, when the thicknesses D1 and D2 aresubstantially equal and Z1 is sufficiently greater than Z2, thepotential difference applied to the second photoelectric conversionlayer 64 b is relatively low, and the magnitude E2 of the electric fieldshown by the above-mentioned Expression (2) may fall below a thresholdvalue E_(TH). That is, when the potential difference Φ applied acrossthe pixel electrode and the opposite electrode is relatively small, thecharges generated in the first photoelectric conversion layer 64 a areselectively detectable. On the other hand, when the potential differenceΦ is relatively large, both the charges generated in the firstphotoelectric conversion layer 64 a and the charges generated in thesecond photoelectric conversion layer 64 b are detectable.

For instance, a case is assumed in which a material having a highabsorption coefficient in a first wavelength range and a material havinga high absorption coefficient in a second wavelength range are used asthe first material and the second material, respectively. For instance,when the visible range is selected as the first wavelength range and theinfrared range is selected as the second wavelength range, when thepotential difference Φ is relatively high, the photoelectric converterhas sensitivity to the visible range and the infrared range. On theother hand, when the potential difference Φ is relatively low, thephotoelectric converter has sensitivity to the visible range. In thepresent description, the infrared range refers to a wavelength range ofapproximately 750 nm or greater, and particularly, near-infrared rangerefers to a wavelength range of 750 nm or greater and less than 2500 nm,for instance.

In this manner, the photoelectric conversion structure 64 is amultilayer structure having the first photoelectric conversion layer 64a and the second photoelectric conversion layer 64 b, and appropriatematerials are used as the first material and the second materialaccording to a wavelength range to be detected. Thus, the spectralsensitivity characteristic of the photoelectric conversion structure 64can be changed by changing the potential difference Φ. It is to be notedthat the above-mentioned magnitude relationship of the potentialdifference Φ indicates relative relationship, and a relatively highpotential difference Φ may be less than 10 V.

In the configuration illustrated to FIG. 3, reset is executed usingreset voltages having different absolute values between the pixelelectrodes 61 x and 61 y while the opposite electrode voltage V_(OPP) isused in common. Thus, at the start of a charge accumulation period, thebias voltage applied to the photoelectric conversion structure is madedifferent between the imaging cell 10Ax and the imaging cell 10Ay.Therefore, different spectral sensitivity characteristics are obtainedbetween the imaging cell 10Ax and the imaging cell 10Ay.

As described later, it is possible to change the spectral sensitivitycharacteristic of the photoelectric converter according to the potentialdifference Φ by setting the ionization potential of the first materialto be greater than the ionization potential of the second material by0.2 eV or more. The details of the photoelectric conversion structurewill be described later.

(Device Structure of Imaging Cell)

Here, the details of the device structure in an imaging cell will bedescribed. FIG. 4 schematically illustrates an exemplary devicestructure of the imaging cell 10Ax or 10Ay included in the pixel arrayPA. The device structure of the imaging cell 10Ay illustrated in FIG. 3is essentially the same as the device structure of the imaging cell10Ax. Thus, the device structure is described using the imaging cell10Ax as an example. FIG. 4 schematically illustrates the arrangement ofthe units included in the imaging cell. The dimension in the unitsillustrated in FIG. 4 does not necessarily match the dimension in anactual device. This applies to other drawings of the present disclosure.

As illustrated in FIG. 4, the imaging cell 10Ax includes part of thesemiconductor substrate 50 that supports the photoelectric converter PC.The semiconductor substrate 50 is not limited to a substrate which is asemiconductor in its entirety, and may be an insulation substrate inwhich a semiconductor layer is provided on the surface on which thephotoelectric converter PC is disposed. Hereinafter, P-type silicon (Si)substrate is exemplified as the semiconductor substrate 50. Theinterlayer insulation layer 52 positioned between the semiconductorsubstrate 50 and the photoelectric converter PCx is typically a silicondioxide layer, and may have a multilayer structure of multipleinsulation layers. The photoelectric conversion structure 64 x in thephotoelectric converter PCx includes a multilayer structure having thefirst photoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b.

The semiconductor substrate 50 has impurity regions (here, N-typeregions) 50 ax to 50 ex, and a device separation region 50 s forelectrical separation from adjacent another imaging cell (the imagingcell 10Ax or 10Ay). The impurity regions 50 ax to 50 ex are typicallydiffusion layers formed in the semiconductor substrate 50. The deviceseparation region 50 s is formed by, for instance, ion implantation ofdonor on predetermined implantation conditions. In this example, thedevice separation region 50 s is also provided between the impurityregion 50 ax and the impurity region 50 bx. It is to be noted that thecenter-to-center distance between two adjacent imaging cells in the rowdirection or the column direction may be on the order of 2 μm, forinstance.

The above-described signal detection transistor 21 includes the impurityregions 50 bx and 50 cx formed in the semiconductor substrate 50, and agate electrode 21 g positioned above the region between the impurityregions 50 b and 50 c out of the major surface of the semiconductorsubstrate 50. The gate electrode 21 g is typically a polysiliconelectrode. The impurity regions 50 bx and 50 cx respectively serve asthe drain region and the source region of the signal detectiontransistor 21. Although not illustrated in FIG. 4, the impurity region50 bx is connected to the power source wire 36 (see FIG. 2).

As schematically illustrated in FIG. 4, the gate electrode 21 g of thesignal detection transistor 21 is connected to the pixel electrode 61 xand the impurity region 50 ax via the connector 54 x disposed in theinterlayer insulation layer 52. In the example illustrated, theconnector 54 x includes a contact plug 54 ax having one end connected tothe gate electrode 21 g, a contact plug 54 b having one end connected tothe impurity region 50 a, a wiring layer 54 c, and a plug 54 d. Thewiring layer 54 c connects the other ends of the contact plugs 54 a and54 b. The pixel electrode 61 x, the connector 54 x, and the impurityregion 50 ax form at least part of the charge accumulation region of theimaging cell 10Ax.

The contact plugs 54 a, 54 b and the wiring layer 54 c are typicallycomposed of polysilicon. The plug 54 d disposed between the wiring layer54 c and the pixel electrode 61 x is composed of copper, for instance.It is to be noted that in addition to the connector 54 s, a wiring layer56 including the vertical signal line 34 (see FIG. 2) is disposed in theinterlayer insulation layer 52. The number of insulation layers in theinterlayer insulation layer 52, and the number of layers included inwiring layers 56 disposed in the interlayer insulation layer 52 can beset to any number.

In the semiconductor substrate 50, not only the signal detectiontransistor 21, but also other transistors, such as the addresstransistor 23, and the reset transistor 22 are also formed. The addresstransistor 23 includes the impurity regions 50 cx and 50 dx, and a gateelectrode 23 g positioned above the region between the impurity regions50 c and 50 d out of the major surface of the semiconductor substrate50. The gate electrode 23 g is typically a polysilicon electrode. Thegate electrode 23 g is connected to the address signal line 39 (notillustrated in FIG. 4, see FIG. 2). The impurity regions 50 cx and 50 dxrespectively serve as the drain region and the source region of theaddress transistor 23. Here, the address transistor 23 is electricallyconnected to the signal detection transistor 21 by sharing the impurityregion 50 cx with the signal detection transistor 21. The impurityregion 50 dx is connected to the vertical signal line 34 via a plugwhich is not illustrated in FIG. 4.

The reset transistor 22 includes the impurity regions 50 ax and 50 ex,and a gate electrode 22 g which is positioned above the region betweenthe impurity regions 50 ax and 50 ex and is connected to the resetsignal line 38 (see FIG. 2). The gate electrode 22 g is also typically apolysilicon electrode. The impurity region 50 ax serves as one of thedrain region and the source region of the reset transistor 22, and theimpurity region 50 ex serves as the other of the drain region and thesource region of the reset transistor 22. Here, the impurity region 50ex is connected to the first signal line 31.

As described above, the photoelectric converter PCx includes the pixelelectrode 61 x, the opposite electrode 62 x, and the photoelectricconversion structure 64 x interposed between the pixel electrode 61 xand the opposite electrode 62 x. Upon receiving incident light, thephotoelectric conversion structure 64 x generates positive and negativecharges (typically positive hole-electron pairs) by photoelectricconversion. When a potential difference is applied across the pixelelectrode 61 x and the opposite electrode 62 x, positive and negativecharges are moved according to the electric field generated between thepixel electrode 61 x and the opposite electrode 62 x. For instance, itis assumed that the potential of the opposite electrode 62 x is higherthan the potential of the pixel electrode 61 x, and the potentialdifference (13 x between the opposite electrode 62 x and pixel electrode61 x is high to some extent. In this situation, the positive charges outof the positive and negative charges generated in the photoelectricconversion structure 64 x can be collected by the pixel electrode 61 x,and the collected charges can be accumulated in the charge accumulationnode FDx as signal charges.

The opposite electrode 62 x of the photoelectric converter PCx istypically a transparent electrode composed of a transparent conductivematerial. Therefore, the opposite electrode 62 having the oppositeelectrodes 62 x and 62 y is typically a transparent electrode. Theopposite electrode 62 x is arranged on the light incident side of thephotoelectric conversion structure 64 x. In other words, the oppositeelectrode 62 x is positioned more distant from the semiconductorsubstrate 50 than the pixel electrode 61 x. For instance, transparentconducting oxide (TCO), such as ITO, IZO, AZO, FTO, SnO₂, TiO₂, ZnO₂ maybe used as the material of the opposite electrode 62 x. It is to benoted that “transparent” in the present description means that at leastpart of light with a wavelength range to be detected can transmit, andit is not required that light can transmit for the entire wavelengthrange of visible light. In the present description, visible light refersto light with a wavelength in a range of 380 nm or greater and less than750 nm. In the present description, entire electromagnetic waveincluding infrared light and ultraviolet rays is represented by “light”for the sake of convenience. The light detected by the imaging device inthe present disclosure is not limited to the light with a wavelengthrange of visible light.

In this example, the opposite electrode 62 that includes, in part, theopposite electrodes 62 x and 62 y forms a single continuous electrodebetween multiple imaging cells. Therefore, it is possible to apply anopposite electrode voltage with a desired amplitude collectively fromthe voltage supply circuit 42 to the opposite electrode 62 x and 62 y ofthe multiple imaging cells via the accumulation control line 35.

In contrast, the pixel electrode 61 x is provided independently for eachimaging cell 10Ax. The pixel electrode 61 x is spatially separated fromthe pixel electrode (the pixel electrode 61 x or 61 y) of anotheradjacent imaging cell, and thus is electrically separated from the pixelelectrode of another imaging cell. The pixel electrode 61 x is composedof metal such as aluminum, copper, and titanium, metal nitride, orpolysilicon to which conductivity is imparted by doping impurities. Thepixel electrode 61 x may be a single electrode or may include multipleelectrodes. For instance, TiN or TaN may be used as the material for thepixel electrode 61 x, which may serve as a light-blocking electrode.

The signal charges (for instance, positive holes) collected by the pixelelectrode 61 x are accumulated in the charge accumulation regionincluding the charge accumulation node FD. The accumulation of signalcharges in the charge accumulation node FDx causes a voltage accordingto the amount of the accumulated signal charges to be applied to thegate of the signal detection transistor 21 of the charge detector CDx. Avoltage amplified by the signal detection transistor 21 is selectivelyread in the form of signal voltage via the address transistor 23. It isto be noted that the charge accumulation region may include a capacitor,for instance. That is, the charge detector CDx may include not only theimpurity region 50 ax in which signal charges are accumulated, but alsoa capacitor (not illustrated in FIG. 4) in which signal charge can beaccumulated, for instance. This also applies to the charge detector CDyof the imaging cell 10Ay.

In the example illustrated in FIG. 4, a color filter 72 and a microlens74 are disposed above the opposite electrode 62 x. As a replacement forthe color filter 72 or along with the color filter 72, an infraredtransmissive filter, a protection layer, or the like is disposed betweenthe microlens 74 and the opposite electrode 62 x. It is not required todispose a microlens 74 corresponding to each of the imaging cells 10Axand Ay. For instance, as illustrated in FIG. 5, one microlens may beshared by the imaging cells 10Ax and Ay.

An imaging device 100T illustrated in FIG. 5 includes a pair of adjacentimaging cells 10Ax and Ay, and one microlens 74 and one color filter 72are shared between these imaging cells 10Ax and 10Ay. The pixel value ofone pixel in an image may be determined by an image signal obtained by apair of these imaging cells.

For instance, it is assumed that the imaging cell 10Ax outputs an imagesignal based on visible light and infrared light, and the imaging cell10 y outputs an image signal based on visible light. In this case, wheneach of pairs of the imaging cells 10Ax and 10Ay included in the pixelarray is associated with each pixel, the output of the imaging cell 10Ayis extracted from each pair, and the pixel value of each pixel isdetermined, then an image based on visible light can be formed.Alternatively, when the pixel value of each pixel is determined by thedifference between the output of the imaging cell 10Ax and the output ofthe imaging cell 10Ay, an image based on visible light can be formed.Although image signals obtained from both of the imaging cells 10Ax and10Ay arranged close to each other in the pixel array are signals basedon light with different wavelength ranges, synchronization isguaranteed. That is, it is possible to obtain multiple signals based onlight with different wavelength ranges.

FIGS. 6 to 8 illustrate an example of the shape of pixel electrodes 61 xand 61 y. In the embodiment of the present disclosure, the pixelelectrodes 61 x and 61 y may have any shape when viewed in a normaldirection of the semiconductor substrate 50. For instance, asillustrated in FIG. 6, the contours of the pixel electrodes 61 x and 61y may be right triangles, and the pixel electrodes 61 x and 61 y may bearranged so that hypotenuses face each other. A single microlens and/ora color filter with a certain color may be disposed so as to cover thesepixel electrodes 61 x and 61 y. The areas of the pixel electrodes 61 xand 61 y may be the same, and may be different. As illustrated in FIG.7, the contours of the pixel electrodes 61 x and 61 y may be rectangles,and the pixel electrodes 61 x and 61 y may be arranged in a squareregion. Alternatively, as illustrated in FIG. 8, as the geometry of thepixel electrodes 61 x and 61 y, one of the electrodes 61 x and 61 y maysurround the other. The contour of the “pixel” defined by a pair ofimaging cells 10Ax and 10 y is not limited to the square as illustrated.

Variation of First Embodiment

FIG. 9 illustrates an exemplary circuit configuration of an imagingdevice according to a variation of the first embodiment. The pixel arrayPA of an imaging device 100B illustrated in FIG. 9 has multiple imagingcells each including at least one pair of the imaging cells 10Bx and10By. Similarly to FIG. 2, in FIG. 9, a pixel block including fourimaging cells arranged in two rows by two columns among the multipleimaging cells is taken and illustrated. In this example, two imagingcell 10Bx are arranged in the first column of the pixel block, and twoimaging cells 10By are arranged in the second column.

As illustrated in FIG. 9, the imaging device 100B has an invertingamplifier for each column of the multiple imaging cells. First,attention is focused on the first column of the pixel block. Aninverting amplifier 49 x is arranged in the first column of the pixelblock, and the inverting input terminal is connected to the verticalsignal line 34 of the first column. That is, the inverting inputterminal of the inverting amplifier 49 x receives the output of theimaging cell 10Bx positioned in the first column. The output terminal ofthe inverting amplifier 49 x is connected to a feedback line 33 x. Inthis example, the source of the reset transistor 22 of the imaging cell10Bx is connected to the feedback line 33 x instead of the first voltageline 31. As seen from FIG. 9, one of the imaging cell 10Bx belonging tothe first column is selected, and the reset transistor 22 and theaddress transistor 23 are turned on, thereby forming a feedback loop forcausing negative feedback of the signal generated by the photoelectricconverter PCx of the imaging cell 10Bx. The effect of random noise canbe canceled by utilizing the negative feedback as described inInternational Publication No. WO 2014/024581.

Next, when attention is focused on the second column of the pixel block,an inverting amplifier 49 y having an inverting input terminal connectedto the vertical signal line 34 of the second column is disposed in thesecond column of the pixel block. The output terminal of the invertingamplifier 49 y is connected to the feedback line 33 y which is connectedto the source of the reset transistor 22 of the imaging cell 10By.

In this example, the first voltage line 31 is connected to thenon-inverting input terminal of the inverting amplifier 49 x in thefirst column, and the second voltage line 32 is connected to thenon-inverting input terminal of the inverting amplifier 49 y in thesecond column. Here, the voltage supply circuit 41 a supplies areference voltage V_(REF) 1 to the non-inverting input terminal of theinverting amplifier 49 x via the first voltage line 31. In contrast, thevoltage supply circuit 41 b supplies a reference voltage V_(REF) 2 tothe non-inverting input terminal of the inverting amplifier 49 y via thesecond voltage line 32. Here, the reference voltage V_(REF) 2 has anabsolute value different from the absolute value of the referencevoltage V_(REF) 1.

As described later, in the configuration illustrated to FIG. 9, thevoltage level of the pixel electrode that gives a level of image signalat a dark time is dependent on and determined by the reference voltageV_(REF) applied to the non-inverting input terminal of the invertingamplifier disposed in each column. That is, in this example, at the timeof operation of the imaging device 100B, the voltage supply circuit 41supplies the reference voltage V_(REF) 1 for resetting the potential ofthe charge accumulation node FDx to the inverting amplifier 49 x in thefirst column via the first signal line 31. Also, the voltage supplycircuit 41 supplies the reference voltage V_(REF) 2 for resetting thepotential of the charge accumulation node FDy to the inverting amplifier49 y in the second column via the second signal line 32.

FIG. 10 selectively illustrates the imaging cells 10Bx and 10By adjacentin the row direction in the pixel array PA illustrated in FIG. 9. Asillustrated in FIG. 10, the charge detector CDx of the imaging cell 10Bxincludes a reset circuit RSx2. The reset circuit RSx2 has a feedbackcircuit FCx2 including the inverting amplifier 49 x. The inverting inputterminal of the inverting amplifier 49 x is electrically connected tothe source of the signal detection transistor 21 via the addresstransistor 23. The feedback circuit FCx2 forms a feedback loop, therebycausing negative feedback of the electrical signals generated in thephotoelectric converter PCx. The charge detector CDy of the imaging cell10By includes a reset circuit RSy2. The reset circuit RSy2 has afeedback circuit FCy2 including the inverting amplifier 49 y. Theinverting input terminal of the inverting amplifier 49 y is electricallyconnected to the source of the signal detection transistor 21 via theaddress transistor 23. The feedback circuit FCy2 forms a feedback loop,thereby causing negative feedback of the electrical signals generated inthe photoelectric converter PCy.

Here, an overview of noise cancellation utilizing negative feedback willbe described. Noise cancellation utilizing negative feedback is executedbefore signal charges are accumulated in a charge accumulation region.Such an operation is also called an “electronic shutter”. Thus, thepotential of the pixel electrode after the noise cancellation gives thevoltage level of an image signal at a dark time.

In the configuration illustrated to FIGS. 9 and 10, reset and noisecancellation are executed as follows. Here, the operation of reset andnoise cancellation is described with attention focused on the imagingcell 10Bx.

First, the reset transistor 22 and the address transistor 23 are turnedon. When the reset transistor 22 and the address transistor 23 areturned on, a feedback loop including the inverting amplifier 49 x in thepath is formed. Due to the formation of a feedback loop, the voltage ofthe charge accumulation node FDx converges to a voltage that causes zerodifference between the voltage of the vertical signal line 34, and thevoltage applied to the non-inverting input terminal of the invertingamplifier 49 x. Here, the non-inverting input terminal of the invertingamplifier 49 x is connected to the first signal line 31. Therefore, atthe time of formation of a feedback loop, the voltage of the chargeaccumulation node FDx converges to a voltage that equate the voltage ofthe vertical signal line 34, and the reference voltage V_(REF) 1. Thatis, the reset circuit RSx2 resets the potential of the pixel electrode61 x to a predetermined potential.

Subsequently, the reset transistor 22 is turned off and noisecancellation is executed. At this point, the potential of the resetsignal line 38 (not illustrated in FIG. 10, see FIG. 9) connected to thegate of the reset transistor 22 is gradually reduced from a high levelto a low level to cross a threshold voltage of the reset transistor 22.When the potential of the reset signal line 38 is gradually reduced froma high level to a low level, the reset transistor 22 gradually changesfrom an ON state to an OFF state. While the reset transistor 22 is on,the formation of a feedback loop is maintained. In this situation, asthe voltage applied to the reset signal line 38 decreases, theresistance of the reset transistor 22 increases. When the resistance ofthe reset transistor 22 increases, the operation range of the resettransistor 22 is reduced, and the frequency domain of returned signalsis reduced.

When the voltage applied to the reset signal line 38 reaches a lowlevel, the reset transistor 22 is turned off. In short, formation of afeedback loop is eliminated. The reset transistor 22 is turned off withthe operation range of the reset transistor 22 lower than the operationrange of the signal detection transistor 21, and thus kTC noise whichremains in the charge accumulation node FDx can be reduced. At thistime, the potential of the charge accumulation node FDx is at apredetermined potential. After the reduction of the kTC noise remainedin the charge accumulation node FDx, the address transistor 23 is turnedoff, and accumulation of signal charges is started.

As is clear from the description of the aforementioned operation, withthe configuration illustrated in FIG. 10, the voltage level of the pixelelectrode 61 x, which gives a level of image signal at a dark time ofeach imaging cell 10Bx belonging to the first column, is dependent onand determined by the reference voltage V_(REF) 1 applied to the firstvoltage line 31. Since the operations of reset and noise cancellationitself are in common between the first column and the second column, thevoltage level of the pixel electrode 61 y, which gives a level of imagesignal at a dark time of each imaging cell 10By belonging to the secondcolumn is dependent on and determined by the reference voltage V_(REF) 2applied to the second voltage line 32. Here, the absolute value of thereference voltage V_(REF) 2 is different from the absolute value of thereference voltage V_(REF) 1. Therefore, the reset circuit RSy2 of theimaging cell 10By resets the potential of the pixel electrode 61 y to apotential different from the potential after the reset of the pixelelectrode 61 x of the imaging cell 10Bx.

Since a common opposite electrode voltage V_(OPP) is applied to theopposite electrode 62 x and the opposite electrode 62 y, the oppositeelectrode 62 x and the opposite electrode 62 y have the same potential.Therefore, the relationship of Φx≠Φy holds at the start of the chargeaccumulation period for the potential difference Φx=(V_(OPP)−V_(REF) 1)between the opposite electrode 62 x and the pixel electrode 61 x, andthe potential difference Φy=(V_(OPP)−V_(REF) 2) between the oppositeelectrode 62 y and the pixel electrode 61 y.

In this manner, the pixel electrode 61 x of the imaging cell 10Bx andthe pixel electrode 61 y of the imaging cell 10By can be reset todifferent potentials by making the reference voltage V_(REF) differentbetween the imaging cells 10Bx and 10By, the bias voltage applied to thephotoelectric conversion structure at the start of a charge accumulationperiod can be made different between the imaging cell 10Bx and theimaging cell 10By. Therefore, the imaging cells with different spectralsensitivity characteristics can be mixed in the pixel array. As in theexample illustrated in FIG. 9, when the inverting amplifiers, to whichthe reference voltages V_(REF) 1 and V_(REF) 2 with different absolutevalues are respectively supplied, are alternately disposed column bycolumn in the multiple imaging cells, an output, in which an imagesignal based on light with a certain wavelength range, and an imagesignal based on light with another wavelength range are interleavedcolumn by column, is obtained.

Second Embodiment

Instead of making the potential of the pixel electrode immediatelybefore the start of accumulation of signal charges different between themultiple imaging cells as in the embodiment described above, thepotential of the opposite electrode may be made different between themultiple imaging cells. As illustrated in FIG. 1, the opposite electrode62 of the photoelectric converter PC is typically formed over themultiple imaging cells. However, it is not required for the imagingdevice in the present disclosure to form the opposite electrode 62 overthe multiple imaging cells.

FIG. 11 illustrates an exemplary circuit configuration of an imagingcell of an imaging device according to a second embodiment of thepresent disclosure. FIG. 11 illustrates the circuit configuration of twoimaging cells taken from the imaging cells included in the pixel arrayPA.

An imaging device 100C illustrated in FIG. 11 has imaging cells 10Cx and10Cy. Similarly to the example described with reference to FIG. 3, thecharge detector CDx of the imaging cell 10Cx and the charge detector CDyof the imaging cell 10Cy respectively include reset circuits RSx1 andRSy1 that each include the reset transistor 22. Here, the potential ofthe pixel electrode 61 of the imaging cell 10Hx and the potential of thepixel electrode 61 of the imaging cell 10Hy after the reset are thesame. In other words, in this example, the potential of the pixelelectrode 61 x of the imaging cell 10 cx and the potential of the pixelelectrode 61 y of the imaging cell 10 cy after the reset are the same.

A photoelectric converter PCx of the imaging cell 10Cx has an oppositeelectrode 62 xs, and a photoelectric converter PCy of the imaging cell10Cy has an opposite electrode 62 ys electrically separated from theopposite electrode 62 xs. As illustrated in FIG. 11, the oppositeelectrode 62 xs and the opposite electrode 62 ys are connected to thefirst voltage line 35 a and the second voltage line 35 b, respectively.The first voltage line 35 a and the second voltage line 35 b areconnected to the voltage supply circuit 42.

In the configuration illustrated to FIG. 11, the voltage supply circuit42 includes voltage supply circuits 42 a and 42 b. Here, the firstvoltage line 35 a and the second voltage line 35 b are connected to thevoltage supply circuits 42 a and 42 b, respectively. The voltage supplycircuit 42 a supplies a first opposite electrode voltage V_(OPP) 1 tothe first voltage line 35 a. The voltage supply circuit 42 b supplies asecond opposite electrode voltage V_(OPP) 2 to the second voltage line35 b. That is, in this example, the imaging device 100C is configured toallow different voltages to be independently applied to the oppositeelectrode 62 xs connected to the first voltage line 35 a, and theopposite electrode 62 ys connected to the second voltage line 35 b. Thevoltage supply circuits 42 a and 42 b may be separate independentvoltage supply circuits, or may be part of a single voltage supplycircuit.

At the time of operation of the imaging device 100C, each of theopposite electrode voltage V_(OPP) 1 and the opposite electrode voltageV_(OPP) 2 may be fixed to a constant value, or may vary periodically orquasi-periodically. For instance, in the charge accumulation period ofan imaging cell 10Cx, the opposite electrode voltage V_(OPP) 1 may besupplied to the first voltage line 35 a, and in the charge accumulationperiod of the imaging cells 10Cy belonging to the same row as theimaging cell 10Cx, the opposite electrode voltage V_(OPP) 2 with anabsolute value different from the absolute value of the oppositeelectrode voltage V_(OPP) 1 may be supplied to the second voltage line35 b. The opposite electrode voltage applied to the opposite electrodein the charge accumulation is made different between the imaging cell10Cx and the imaging cell 10Cy, and thus the potential of the oppositeelectrode 62 xs of the imaging cell 10Cx, and the potential of theopposite electrode 62 ys of the imaging cell 10Cy can be made differentin the charge accumulation. For instance, when a common reset voltage isused as in this example, the bias voltage applied to the photoelectricconversion structure in the charge accumulation can be made differentbetween the imaging cell 10Cx and the imaging cell 10Cy. Consequently,the sensitivity of the imaging cell 10Cx and the sensitivity of theimaging cell 10Cy can be made different.

In this manner, separation of the opposite electrode 62 between theimaging cell 10Cx and the imaging cell 10Cy allows the oppositeelectrode voltages with different absolute values to be independentlyapplied to the opposite electrode 62 xs of the imaging cell 10Cx and theopposite electrode 62 ys of the imaging cell 10Cy. Consequently,similarly to the first embodiment, the imaging cells with differentspectral sensitivity characteristics can be mixed in the pixel array PA.

For instance, by applying photolithography, the opposite electrode 62 xsand the opposite electrode 62 ys can be formed with spatially separated,and these opposite electrodes can be electrically separated. Forinstance, multiple opposite electrodes may be formed in which theimaging cells are spatially separated column by column. In this case,multiple strip-shaped opposite electrodes each extending in the columndirection are arranged in the row direction, and the first voltage line35 a and the second voltage line 35 b may be alternately connected tothese multiple opposite electrodes. Specifically, the imaging cells 10Cxand 10Cy are arranged in, for instance, the odd columns and the evencolumns of the pixel array PA, respectively, and different oppositeelectrode voltages may be applied to the odd columns and the evencolumns of the pixel array PA. In this case, for instance, an output, inwhich an image signal based on light with a certain wavelength range,and an image signal based on light with another wavelength range areinterleaved column by column, is obtained. Alternatively, the oppositeelectrodes may be electrically separated row by row in the multipleimaging cells. In this case, multiple strip-shaped opposite electrodeseach extending in the row direction are formed in the column direction,and the first voltage line 35 a and the second voltage line 35 b may bealternately connected to these multiple opposite electrodes. The latteris more advantageous than the former from the viewpoint of reduction inthe number of wires per row.

In the latter case, the imaging cells 10Cx and 10Cy are adjacent to eachother in the column direction. Therefore, in this case, the start timingof the charge accumulation period of the imaging cell 10Cx does notmatch the start timing of the charge accumulation period of the imagingcell 10Cy. For instance, when a rolling shutter operation is applied,the start timing of the charge accumulation period of the imaging cell10Cx belonging to a certain row may not match the start timing of thecharge accumulation period of the imaging cell 10Cx belonging to anotherrow. However, even in this case, the vertical signal line 34 isindependently provided to each of the imaging cell 10Cx and the imagingcell 10Cy that are adjacent in the column direction, and thus the starttimings of the charge accumulation periods between these imaging cellscan be matched.

The opposite electrode voltages V_(OPP) 1 and V_(OPP) 2 to be suppliedto the first voltage line 35 a and the second voltage line 35 b,respectively in a period other than the charge accumulation period maybe the same. In a period other than the charge accumulation period, forinstance, a period for reset, an opposite electrode voltage V_(OPP)substantially equivalent to the reset voltage VRST may be applied incommon to the first voltage line 35 a and the second voltage line 35 b.Consequently, in the period, the first bias voltage V1 applied to thephotoelectric conversion structure 64 x, and the second bias voltage V2applied to the photoelectric conversion structure 64 y can be a valuenear 0 V. When the potential difference Φ applied across a pair ofelectrodes, by which the photoelectric conversion structure isinterposed, is substantially 0 V, even when light enters thephotoelectric conversion structure, the positive charges generated inthe photoelectric conversion structure are not collected by the pixelelectrode. Also, backflow of charge from the charge accumulation regionto the photoelectric conversion structure hardly occurs. That is, astate can be achieved in which substantially no charges are movedbetween the photoelectric conversion structure and the electrodes bymaking the potential difference Φ nearly 0 V, and even when thephotoelectric conversion structure is irradiated with light, thepotential of the charge accumulation node hardly changes and thesensitivity is substantially zero. In other words, a state with ashutter closed can be electrical control. The details of theconfiguration of the photoelectric conversion structure 64, which givessuch a characteristic, will be described later.

Third Embodiment

FIG. 12 illustrates an exemplary circuit configuration of an imagingdevice according to a third embodiment of the present disclosure. Apixel array PA of imaging devices 100D illustrated in FIG. 12 hasmultiple imaging cells each including at least one pair of the imagingcells 10Dx and 10Dy.

Similarly to FIGS. 2 and 9, in FIG. 12, a pixel block including fourimaging cells arranged in two rows by two columns among the multipleimaging cells is taken and illustrated. The pixel block illustrated inFIG. 12 has two imaging cells 10Dx and two imaging cells 10Dy. In thisexample, the imaging cells 10Dx and 10Dy are adjacent to each other inthe row direction as well as in the column direction.

The circuit configuration of imaging cell 10Dx is substantially the sameas the circuit configuration of imaging cell 10Ax, which has beendescribed with reference to FIG. 2. However, in this example, the sourceof the reset transistor 22 of the charge detector CDx is not connectedto the first voltage line 31 but is connected to a reset voltage line 37r that receives supply of the reset voltage VRST at the time ofoperation of the imaging device 100D.

The source of the reset transistor 22 of the charge detector CDy of theimaging cell 10Dy is also connected to the reset voltage line 37 r.Therefore, in this example, the potential of the pixel electrode of theimaging cell 10Dy after reset is the same as the potential of the pixelelectrode of the imaging cell 10Dx after reset.

The charge detector CDy of the imaging cell 10Dy has a capacitor 25 yhaving one of electrodes connected to charge accumulation node FDy, inaddition to the signal detection transistor 21, the reset transistor 22,and the address transistor 23. The other electrode of the capacitor 25 yis connected to the first voltage line 31 which is connected to voltagesupply circuit 41. That is, the capacitor 25 y is connected between thefirst voltage line 31 and the pixel electrode of the photoelectricconverter PCy. It is to be noted that in this example, a parasiticcapacitance may exist between the first voltage line 31 and the chargeaccumulation node FDx, however, the capacitive value of the parasiticcapacitance is so small to be neglectable compared with the capacitivevalue of the charge accumulation node FDx. That is, the first signalline 31 has no intentional electrical coupling between the imaging cell10Dx and itself. In the configuration illustrated to FIG. 12, thevoltage supply circuit 41 selectively supplies a voltage V_(TP) to thecapacitor 25 y of the imaging cell 10Dy via the first voltage line 31.In the following description, the voltage V_(TP) may be called the“offset voltage V_(TP)”.

FIG. 13 selectively illustrates the imaging cells 10Dx and 10Dy adjacentin the row direction in the pixel array PA illustrated in FIG. 12. Asillustrated in FIG. 13, here, the opposite electrode 62 x of the imagingcell 10Dx and the opposite electrode 62 y of the imaging cell 10Dy areconnected to a common accumulation control line 35 to which the oppositeelectrode voltage V_(OPP) is applied. Therefore, the potential of theopposite electrode 62 x in a charge accumulation period is equal to thepotential of the opposite electrode 62 y in the charge accumulationperiod. Also, the source of the reset transistor 22 of each of thecharge detectors CDx and CDy is connected to the common reset voltageline 37 r. Therefore, the reset circuit RSx1 of the imaging cell 10Dxresets the potential of the pixel electrode 61 x to the potentialaccording to the reset voltage VRST supplied to the reset voltage line37 r. The reset circuit RSy1 of the imaging cell 10Dy resets thepotential of the pixel electrode 61 y to the same potential as thepotential of the pixel electrode 61 x after the reset.

In the configuration illustrated to FIG. 13, the capacitor 25 y isconnected between the pixel electrode 61 y of the imaging cell 10Dy andthe first voltage line 31, thereby electrically coupling the pixelelectrode 61 y of the imaging cell 10Dy and the first voltage line 31via the capacitor 25 y. Due to the electrical coupling between the pixelelectrode 61 y and the first voltage line 31 connected to the voltagesupply circuit 41, the potential of the charge accumulation node FDyincluding pixel the electrode 61 y can be changed by changing thevoltage of the first voltage line 31.

The voltage supply circuit 41 may be configured to switch between atleast two voltages with different absolute values and to allow a voltageto be applied to the first signal line 31. The voltage supply circuit 41may apply a voltage higher in a charge accumulation period than in otherperiods included in a frame to the first voltage line 31. That is, thevoltage supply circuit 41 temporarily increases the offset voltageV_(TP) to be supplied to the first voltage line 31 in the chargeaccumulation period in the frame period. The offset voltage V_(TP) istemporarily increased in a charge accumulation period, and the potentialof the pixel electrode 61 y thereby can be temporarily increased byelectrical coupling between the first voltage line 31 and the chargeaccumulation node FDy via the capacitor 25 y in a charge accumulationperiod. It is to be noted that the other periods included in the frameperiod are, for instance, a reset period, and a read period.

(Electrical Coupling via Capacitor)

The change in the potential of the pixel electrode 61 y by switching theoffset voltage V_(TP) applied to the first voltage line 31 will bedescribed. For instance, it is assumed that after the potential of thecharge accumulation node FDy is reset, the reset transistor 22 of thecharge detector CDy is turned off, and accumulation of signal charges isstarted by switching the offset voltage V_(TP) from the low-levelvoltage V_(L) to the high-level voltage V_(H). For instance, the powersupply voltage, or a voltage decreased or increased from the powersupply voltage may be used as the voltage V_(H). This also applies tothe voltage V_(L).

The offset voltage V_(TP) to be applied to the first voltage line 31 ischanged from the voltage V_(L) to the voltage V_(H). At this point, thevoltage of the charge accumulation node FDy is changed from the voltageVRST immediately after the reset by electrical coupling between thefirst voltage line 31 and the charge accumulation node FDy via thecapacitor 25 y. The amount of change then in the voltage of the chargestorage node FDy is determined by the following manner.

Let C_(FDy) be the capacitive value of the charge accumulation node FDy,Cy be the capacitive value of the capacitor 25 y, and VRST be thevoltage of the charge accumulation node FDy immediately after the reset.At this point, the charge quantity Q accumulated in the chargeaccumulation node FDy immediately before switching of the offset voltageV_(TP) can be expressed by Q=C_(FDy)VRST−Cy (V_(L)−VRST). Let V_(FD) bethe voltage of the charge accumulation node FDy immediately after theswitching of the offset voltage V_(TP) to the voltage V_(H), thenQ=C_(FDy)V_(FD)−Cy (V_(H)−V_(FD)) holds. When an equation is formed byequating these right-hand sides because of the condition of chargeneutrality, and the equation is solved for V_(FD),V_(FD)=(Cy/(Cy+C_(FDy)))(V_(H)−V_(L))+VRST is obtained. Thus, the amountof change in the voltage of the charge accumulation node FDy byswitching the offset voltage V_(TP) from the voltage V_(L) to thevoltage V_(H) is expressed by the following Expression (3).(Cy/(Cy+C _(FDy)))(V _(H) −V _(L))  (3)

When a capacitive value C1y of the capacitor 25 y is sufficiently largewith respect to C_(FDy), from the above-mentioned Expression (3),Expression (3) approximately gives (V_(H)−V_(L)). That is, the voltageV_(FD) of the charge accumulation node FDy can be changed by an amountapproximately equal to (V_(H)−V_(L)) which is the change in the offsetvoltage V_(TP) by switching of the offset voltage V_(TP). Therefore, thepotential difference Φ between the opposite electrode 62 y and the pixelelectrode 61 y is reduced by an amount approximately equal to(V_(H)−V_(L)) by switching the offset voltage V_(TP) from the voltageV_(L) to the voltage V_(H).

With the electrical coupling via the capacitor 25 y, the change in thevoltage applied to the first voltage line 31 is accompanied by theabove-described change in the potential of the pixel electrode 61 y.When the voltage applied to the first voltage line 31 is returned to theoriginal voltage, the potential of the pixel electrode 61 y is alsoreturned to the voltage before the change of the voltage applied to thefirst voltage line 31. That is, it is possible to change the potentialof the pixel electrode 61 y by selectively changing the voltage appliedto the first voltage line 31 in a charge accumulation period withoutaffecting the charge quantity accumulated in the charge accumulationnode FDy.

In this manner, according to the third embodiment, the bias voltageapplied to the photoelectric conversion structure 64 y can beselectively changed in a charge accumulation period by using the resetvoltage VRST in common between the imaging cells 10Dx and 10Dy. Thus,the bias voltage applied to the photoelectric conversion structure atthe start of a charge accumulation period can be made different betweenthe imaging cells 10Dx and 10Dy. Therefore, for instance, the spectralsensitivity characteristics of the imaging cells 10Dx and 10Dy can bemade different by switching the offset voltage V_(TP) applied to thefirst voltage line 31 between multiple voltages with different absolutevalues.

The capacitor 25 y may be a device having ametal-insulator-semiconductor (MIS) structure, for instance, or may beformed as metal-insulator-metal (MIM) structure in the interlayerinsulation layer 52. The MIM structure refers to a structure in which adielectric substance is interposed between two electrodes which arecomposed of a metal or a metal compound. SiO₂, Al₂O₃, SiN, HfO₂, ZrO₂may be used as the dielectric substance interposed between twoelectrodes. Alternatively, the capacitor 25 y may have a structure inwhich a parasitic capacitance between wires is intentionally utilized.In this case, the capacitive value of the parasitic capacitance betweenthe first voltage line 31 and the charge accumulation node FDy has asignificantly large value compared with the capacitive value of thecharge accumulation node FDy. The capacitor 25 y may include two or morecapacitors.

(Example of Operation of Imaging Device)

Here, the relationship between a frame period and a charge accumulationperiod will be described briefly. FIG. 14 illustrates a typical exampleof an operation of an imaging device when a rolling shutter is appliedto the imaging device according to an embodiment of the presentdisclosure. Although the actual number of rows included in the pixelarray PA may reach several hundreds to several thousand rows, here, thethree rows of the first, second, and third rows are taken and theoperation of the imaging cells is schematically illustrated due tolimitations of space. Here, an example of the operation of the imagingcell 10Dy illustrated in FIG. 13 will be described.

When an image signal is obtained, first, the potential of the chargeaccumulation node FDy is reset. In other words, the reset transistor 22is turned on, and the charge in the charge accumulation node FDy isdischarged. Due to the turning on of the reset transistor 22, thepotential of the pixel electrode 61 y is reset to e level according tothe reset voltage VRST. The voltage level of the pixel electrode 61 ythen determines a level of the image signal at a dark time. The firstreset period corresponds to what is called an electronic shutter. In thefirst reset period, for instance, the low-level voltage V_(L) is appliedto the first voltage line 31. In FIG. 14, a hatched rectangle RT1 at theleft end represents the first reset period.

Next, the reset transistor 22 is turned off. At this point, insynchronization with the turning off of the reset transistor 22, theoffset voltage V_(TP) applied to the first voltage line 31 is switchedto the high-level voltage V_(H), for instance. Accumulation of signalcharges in the charge accumulation node FDy is started by turning offthe reset transistor 22. Subsequently, the address transistor 23 isturned on at a desired timing, the offset voltage V_(TP) is returned tothe low-level voltage V_(L), and a signal is read to the vertical signalline 34. The level of a signal read at this point corresponds to theamount of signal charges accumulated in the charge accumulation regionin the period from the initial reset of the charge accumulation node FDuntil turning on of the address transistor 23. In FIG. 14, a lightlyhatched rectangle RD1 represents a period in which a signal is read(first signal read period) according to the amount of signal chargesaccumulated in the charge accumulation region. In FIG. 14, a whiterectangle EXP represents a period from a state of dark level of thepotential of the charge accumulation node FD until the first signal readperiod, and the period corresponds to the above-described “chargeaccumulation period”. The charge accumulation period is a period inwhich signal charges are essentially accumulated in the chargeaccumulation region, and may also be called an exposure period. Here,the reset transistor 22 is turned off, and the offset voltage V_(TP) isswitched to high-level. Thus, at the time of start of a chargeaccumulation period, the potential difference between the pixelelectrode 61 y and the opposite electrode 62 y is different from thepotential difference between the pixel electrode 61 x and the oppositeelectrode 62 x.

As illustrated in FIG. 14, in synchronization with completion of acharge accumulation period, the offset voltage V_(TP) to be applied tothe first voltage line 31 is switched to the low-level V_(L). Switchingof the offset voltage V_(TP) may be performed simultaneously with theturning on of the address transistor 23. Subsequently, the resettransistor 22 is turned on again, and the potential of the pixelelectrode 61 y is thereby reset again to a level corresponding to thereset voltage VRST. In FIG. 14, a hatched rectangle RT2 represents thereset period following the first signal read period.

Subsequently, the address transistor 23 is turned on again, and a signalafter the reset is read again. Here, the voltage applied to the firstvoltage line 31 from the voltage supply circuit 41 is the low-levelvoltage V_(L), and the level of a signal read here corresponds to alevel at a dark time. Thus, an image signal with fixed noise removed isobtained by calculating the difference between the level of a signalread at this point and the level of a signal read in the first signalread period represented by the rectangle RD1. In FIG. 14, a relativelydark hatched rectangle RD2 represents a period in which a signal is read(second signal read period) after the potential of the chargeaccumulation node is reset. After a signal corresponding to a level at adark time, the address transistor 23 is turned off. The time taken forreading a signal is relatively short, thus the potential of the chargeaccumulation region hardly changes before and after the signal is read.In the present description, the “frame period” indicates the period fromthe start of the charge accumulation period in the start row to the endof a second signal read period in the last row.

Variation of Third Embodiment

FIG. 15 illustrates the circuit configuration of two imaging cells takenfrom the imaging cells included in the pixel array PA of an imagingdevice according to a variation of the third embodiment. Similarly tothe imaging cell 10Bx described with reference to FIG. 10, an imagingcell 10Ex illustrated in FIG. 15 includes the reset circuit RSx2 whichhas the feedback circuit FCx2. An imaging cell 10Ey also has the resetcircuit RSy2 which has the feedback circuit FCy2. However, in thisexample, the non-inverting input terminal of the inverting amplifier 49x in the feedback circuit FCx2 is not connected to the first voltageline 31, and receives supply of a predetermined reference voltageV_(REF) from a voltage line which is not illustrated. Similarly, thenon-inverting input terminal of the inverting amplifier 49 y in thefeedback circuit FCy2 is not connected to the second voltage line 32,and receives supply of a predetermined reference voltage V_(REF) from avoltage line which is not illustrated. The potential of the pixelelectrode 61 x after reset can be substantially the same as thepotential of the pixel electrode 61 y after reset by adjusting thereference voltage V_(REF) and the reference voltage V_(REF)′appropriately. The reference voltage V_(REF) and the reference voltageV_(REF)′ may be the same voltage.

In addition to the reset circuit RSx2, the charge detector CDx of theimaging cell 10Ex has a capacitor 25 x connected between the chargeaccumulation node FDx and the first voltage line 31. That is, in thisexample, the pixel electrode 61 x of the imaging cell 10Ex and the firstvoltage line 31 are electrically coupled to each other via the capacitor25 x. The charge detector CDy of the imaging cell 10Ey includes thecapacitor 25 y connected between the charge accumulation node FDy andthe first voltage line 31. The capacitor 25 x of the charge detector CDxand the capacitor 25 y of the charge detector CDy typically havedifferent capacitive values.

In the example illustrated in FIG. 15, the capacitor 25 x of the chargedetector CDx and the capacitor 25 y of the charge detector CDy are bothconnected to the same first voltage line 31. Therefore, change in thepotential of the side of the electrode, connected to the first voltageline 31 is in common with the capacitor 25 x and the capacitor 25 y.However, here, the capacitive value of the capacitor 25 x and thecapacitive value of the capacitor 25 y of the imaging cell 10Ey aredifferent from each other. Therefore, even though the change in theoffset voltage V_(TP) is in common, the change in the voltage of thecharge accumulation node is different between the imaging cells 10Ex and10Ey. Hereinafter, this point will be described.

Here, for the sake of simplicity, it is assumed that the voltages of thecharge accumulation node FDx and the charge accumulation node FDy afterthe reset are the same voltage. Let C_(FDx) be the capacitive value ofthe charge accumulation node FDx, and Cx be the capacitive value of thecapacitor 25 x, then the amount of change in the voltage of the chargeaccumulation node FDx when the offset voltage V_(TP) applied to thefirst voltage line 31 is switched from the voltage V_(L) to the voltageV_(H) is expressed by the (Cx/(Cx+C_(FDx)))(V_(H)−V_(L)) based on theabove-mentioned Expression (3).

Here, when the capacitive value Cx of the capacitor 25 x is smaller thanthe capacitive value Cy of the capacitor 25 y of the imaging cell 10Ey,and is approximately equal to the C_(FDx), for instance, the amount ofchange in the voltage of the charge accumulation node FDx isapproximately only half of (V_(H)-V_(L)). When the capacitive value Cxis sufficiently smaller than the C_(FDx), it is found from Expression(3) that even if the offset voltage V_(TP) is changed, the voltage ofthe charge accumulation node FDx hardly changes.

In this manner, with the electrical coupling via a capacitor, thecapacitive value of the capacitor, which electrically couples the firstvoltage line 31 to the pixel electrode, is made different between theimaging cells, and thereby the change in the potential difference Φbetween the pixel electrode and the opposite electrode can be madedifferent between the two imaging cells while the offset voltage V_(TP)applied to the first signal line 31 is used in common. It is to be notedthat reducing the capacitive value of the capacitor interposed betweenthe first voltage line 31 and the pixel electrode 61 is advantageous forminiaturization of pixel size. On the other hand, interposing acapacitor having a larger capacitive value between the first voltageline 31 and the pixel electrode 61 allows the potential of the chargeaccumulation node FD to be changed by switching the voltage applied tothe first voltage line 31 without unnecessarily increasing the amount ofchange in the voltage applied to the first voltage line 31.

With the electrical coupling via a capacitor, the change in thepotential difference J between the pixel electrode and the oppositeelectrode can be made different between multiple imaging cells withoutmaking the configuration and wiring of the voltage supply circuitcomplicated. Thus, an imaging cell in which the potential difference Lshows a relatively large change, and an imaging cell in which thepotential difference Φ shows substantially no change for a change in thevoltage applied to the first voltage line 31 can be mixed in the pixelarray. Therefore, it is possible to achieve a state in which forinstance, the photoelectric converter PCy of the imaging cell 10Ey hassensitivity in the wavelength range of visible light, and thephotoelectric converter PCx of the imaging cell 10Ex also hassensitivity to the wavelength range of infrared light in addition to thewavelength range of visible light by switching the offset voltage V_(TP)while the offset voltage V_(TP) applied to the first voltage line 31 isused in common. In other words, the imaging cells with differentspectral sensitivity characteristics can be mixed in the pixel array PAwhile the offset voltage V_(TP) applied to the first voltage line 31 isused in common. The first voltage line 31 may be provided independentlyfor each column or each row of the pixel array PA, and may be connectedto all the imaging cells included in the pixel array PA.

With the electrical coupling via a capacitor, even when the potential ofthe pixel electrode 61 is increased through the change in the voltageapplied to the first voltage line 31 at the time of start of a chargeaccumulation period, when the voltage applied to the first signal line31 is returned to the original voltage, the potential of the pixelelectrode 61 is also returned to the voltage before the change of thevoltage applied to the first signal line 31. Therefore, a signal can beread by changing the voltage applied to the first voltage line 31 in asignal read period back to the voltage applied to the first voltage line31 in a reset period. In other words, this is advantageous from theviewpoint of reduction in power consumption because it is not necessaryto use a higher power supply voltage AVDD to read a signal.

As is seen from Expressions (3), the above-mentioned effect is achievedwhen the ratio of the capacitive value of the capacitor thatelectrically couples the first voltage line 31 and the pixel electrodewith respect to the capacitive value of the charge accumulation node isdifferent between the imaging cells. Thus, the change in the voltage ofthe charge accumulation node can be made different between the imagingcells, while the change in the offset voltage V_(TP) is in common.Therefore, it is not required that the capacitive value of the capacitorthat electrically couples the first voltage line 31 and the pixelelectrode is made different between adjacent imaging cells. Forinstance, the capacitive value of the capacitor that electricallycouples the first voltage line 31 and the pixel electrode 61 may be madecommon between the multiple imaging cells, and the capacitive value ofthe charge accumulation node may be made different between the multipleimaging cells.

For instance, the area of the portion where the pixel electrode and theopposite electrode overlap may be made different between thephotoelectric converter PCx of the imaging cell 10Ex, and thephotoelectric converters PCy of the imaging cell 10Ey. The photoelectricconverter itself has a capacitive, and it can be said that for instance,an imaging cell including a pixel electrode having a larger area is lessaffected by switching of the voltage applied to the first voltage line31.

FIG. 16 illustrates the circuit configuration of two imaging cells takenfrom the imaging cells included in the pixel array PA of the imagingdevice according to another variation of the third embodiment. Incontrast to the example illustrated in FIG. 15, in the exampleillustrated in FIG. 16, instead of the imaging cell 10Ey, the imagingcell 10Fy is arranged adjacent to the imaging cell 10Ex.

The charge detector CDy of the imaging cell 10Fy has a reset circuitRSy3. The reset circuit RSy3 includes a reset transistor 22 a, and afeedback circuit FCy3 having the inverting amplifier 49 y and a feedbacktransistor 22 b. The feedback transistor 22 b is connected between thesource of the reset transistor 22 a, and feedback line 33 y connected toan output terminal of the inverting amplifier 49 y. A common signal linemay be connected to the gate of the feedback transistor 22 b and thegate of the reset transistor 22 of the imaging cell 10Ex. In otherwords, the on and off operations of the feedback transistor 22 b can besimilar to the on and off operations of the reset transistor 22 of theimaging cell 10Ex.

The imaging cell 10Fy includes a capacitive circuit having the capacitor25 y and a second capacitor 25 z between the pixel electrode 61 y andthe first voltage line 31. One of the electrodes of the capacitor 25 zis connected to the pixel electrode 61 y, and the other electrode isconnected to the source of reset transistor 22 a. In short, thecapacitor 25 z is connected in parallel to the reset transistor 22 a. Inthis example, the charge detector CDy includes not only the impurityregion 50 ay (for instance, see FIG. 1), but also the capacitors 25 yand 25 z in part. The capacitor 25 z has a capacitive value smaller thanthe capacitive value of the capacitor 25 y. Accumulation of signalcharges with the offset voltage V_(TP) changed is executed with thereset transistor 22 a off.

An increase in the synthetic capacity of the entire charge accumulationregion can be reduced by connecting the capacitor 25 y to the chargeaccumulation node FDy via the capacitor 25 z. In other words, the effectof the change in the voltage applied to the first voltage line 31 on thepotential difference Φy between the pixel electrode 61 y and theopposite electrode 62 y is reduced. Also, since the capacitive circuitconnected between the pixel electrode 61 y and the first voltage lines31 includes the capacitor 25 z, the effect of noise cancellation can beimproved by reducing the decrease in the conversion gain. Hereinafter,an overview of noise cancellation by the feedback circuit FCy3 will bedescribed.

For instance, the reset of a signal charge after the first signal readperiod is executed as follows. First, a feedback loop is formed byturning on the reset transistor 22 a and the feedback transistor 22 bwith the address transistor 23 on. The formation of a feedback loopcauses negative feedback of an output of the signal detection transistor21. Due to the negative feedback of the output of the signal detectiontransistor 21, the potential of the charge accumulation node FDyconverges a potential such that the voltage of the vertical signal line34 is equalized to the V_(REF)′.

Next, the reset transistor 22 a is turned off. kTC noise occurs byturning off of the reset transistor 22 a. Therefore, the voltage of thecharge accumulation node FDy after the reset includes the kTC noise thataccompanies the turning off of the reset transistor 22 a. After theturning off of the reset transistor 22 a, cancellation of the kTC noiseis executed.

While the feedback transistor 22 b is on, the formation of the feedbackloop is maintained. Thus, the kTC noise generated by turning off thereset transistor 22 a is reduced to the magnitude of 1/(1+A), where A isthe gain of the feedback circuit FCy. In this example, the voltage ofthe vertical signal line 34 immediately before the turning off the resettransistor 22 a, that is, immediately before the start of noisecancellation is approximately equal to the reference voltage V_(REF)′which is applied to the positive-side input terminal of the invertingamplifier 49 y. The kTC noise can be canceled in a relatively short timeby setting the voltage of the vertical signal line 34 at the start ofnoise cancellation to a level close to the reference voltage V_(REF)′.

Next, the feedback transistor 22 b is turned off. The kTC noise occursaccompanying the turning off of the feedback transistor 22 b. However,the size of kTC noise added to the voltage of the charge accumulationnode FDy due to the turning off of the feedback transistor 22 b is(C_(FDy)/Cy)^(1/2)×(Cz/(Cz+C_(FDy))) times greater than the size of kTCnoise when the feedback transistor 22 b is directly connected to thecharge accumulation node FDy without providing the capacitor 25 y andthe capacitor 25 z in the imaging cell 10Fy. In the above Expression, Czindicates the capacitive value of the capacitor 25 z, and “x” indicatesmultiplication.

From the above Expression, it is seen that larger the capacitive valueCy of the capacitor 25 y, smaller the generated noise itself, andsmaller the capacitive value Cz of the capacitor 25 z, lower theattenuation rate. The kTC noise generated by turning off the feedbacktransistor 22 b can be sufficiently reduced by setting the capacitivevalues Cy and Cz appropriately. After the feedback transistor 22 b isturned off, a signal, in which the kTC noise has been canceled, is read.

The capacitor 25 y is connected to the charge accumulation node FDy viathe capacitor 25 z with the reset transistor 22 a and the feedbacktransistor 22 b turned off. Here, it is assumed that the chargeaccumulation node FDy and the capacitor 25 y are directly connected, notvia the capacitor 25 z. In this case, the capacitive value of the signalcharges in the entire accumulation region when the capacitor 25 y isdirectly connected is (C_(FDy)±Cy). That is, when the capacitor 25 y hasa relatively large capacitive value Cy, the capacitive value of thesignal charges in the entire accumulation region also becomes large, anda high conversion gain (may be referred to a high S/N ratio) is notobtained. On the other hand, when the capacitor 25 y is connected to thecharge accumulation node FDy via the capacitor 25 z as illustrated inFIG. 16, the capacitive value of the signal charges in the entireaccumulation region in this configuration is expressed by(C_(FDy)+(CyCz)/(Cy+Cz)). Here, when the capacitor 25 z has a relativelysmall capacitive value Cz, and the capacitor 25 y has a relatively largecapacitive value Cy, the capacitive value of the signal charges in theentire accumulation region is approximately (C_(FDy)+Cz). That is, theincrease in the capacitive value of the signal charges in the entireaccumulation region is small. In this manner, decrease in the conversiongain can be reduced by connecting the capacitor 25 y to the chargeaccumulation node FDy via the capacitor 25 z having a relatively smallcapacitive value.

In addition, the effect is achieved that the change in the potential ofthe charge accumulation node FDy with respect to the change in theoffset voltage V_(TP) is changed by connecting the capacitor 25 y to thecharge accumulation node FDy via the capacitor 25 z having a relativelysmall capacitive value. In the example illustrated in FIG. 16, the pixelelectrode 61 y and the opposite electrode 62 y of the imaging cell 10Fyhas an electrode area larger than the electrode area of the pixelelectrode 61 x and the opposite electrode 62 x of the imaging cell 10Ex.Therefore, the imaging cell 10Fy including the pixel electrode 61 yhaving a larger area is less affected than the imaging cell 10Ex byswitching of the voltage applied to the first voltage line 31.

It can be said that an imaging cell having a relatively low capacitivevalue of the charge accumulation region has a high conversion gain, anda high sensitivity. On the other hand, when the capacitive value of thecharge accumulation region is increased, the conversion gain is reduced,thus it is advantageous for photographing under high illumination.Composing the image data obtained by two imaging cells having differentcapacitive values of the charge accumulation region allows an image withno blown out highlights and blocked up shadows to be formed even for ascene with a large contrast ratio. Such image formation is called “highdynamic range imaging”.

Other Embodiments

FIG. 17 schematically illustrates an exemplary configuration of animaging device according to another embodiment of the presentdisclosure. An imaging device 100E illustrated in FIG. 17 has a pair ofthe imaging cells 10 x and 10 y, and a subtraction circuit 80.

The imaging cell 10 x may be one of the above-described imaging cells10Ax to 10Ex, and the imaging cell 10 y may be one of theabove-described imaging cells 10Ay to 10Fy. Here, the imaging cells 10Exand 10Ey described with reference to FIG. 15 are illustrated as theimaging cells 10 x and 10 y.

The offset voltage V_(TP) supplied to the first voltage line 31 (notillustrated in FIG. 17, see FIG. 15) is selectively increased, forinstance, in a charge accumulation period. Consequently, in the imagingcell 10 y, the absolute value of the potential difference Φy between thepixel electrode 61 y and the opposite electrode 62 y is reduced, and theimaging cell 10 y loses the sensitivity to the wavelength range ofinfrared light, and selectively has sensitivity to the wavelength rangeof visible light. On the other hand, in the imaging cell 10 x, even whenthe offset voltage V_(TP) supplied to the first voltage line 31 isincreased, the absolute value of the potential difference Φx between thepixel electrode 61 x and the opposite electrode 62 x hardly changes, andthus the imaging cell 10 x has sensitivity in the wavelength range ofvisible light and infrared light.

In this example, the imaging cell 10 x has sensitivity in the wavelengthrange of visible light and infrared light in a charge accumulationperiod, and the charge detector CDx outputs a first image signal basedon visible light and infrared light. Therefore, it is possible to forman image based on visible light and infrared light from the first imagesignal obtained by the imaging cell 10 x. On the other hand, the imagingcell 10 y selectively has sensitivity in the wavelength range of visiblelight in the charge accumulation period, and the charge detector CDyoutputs a second image signal based on visible light. Therefore, it ispossible to form an image based on visible light from the second imagesignal obtained by the imaging cell 10 y.

The subtraction circuit 80 is configured to be connected to the chargedetector CDx and the charge detector CDy, and to output the differencebetween the first image signal and the second image signal. It ispossible to form an image based on infrared light by calculating thedifference between the level of an output signal of the imaging cell 10x and the level of an output signal of imaging cell 10 y. The outputsignal of the imaging cell 10 x and/or the output signal of the imagingcell 10 y may be amplified by a predetermined gain, and subtractionprocessing may be performed. In this case, the gain does not need to bematched between the imaging cells 10 x and 10 y. It is sufficient that aratio of gains be determined for each pair of the imaging cells 10 x and10 y.

With the technique described in Japanese Unexamined Patent ApplicationPublication No. 2008-227092, one of a monochrome image and an infraredimage is selectively obtained, thus synchronization between these imagesis guaranteed. Therefore, the technique is not suitable forphotographing an object moving at a high speed, for instance. Incontrast, the first and second image signals obtained from the imagingcells 10 x and 10 y, respectively can be signals based on the signalcharges accumulated in the same charge accumulation period. In otherwords, synchronization between the image based on the first image signaland the image based on the second image signal can be guaranteed. Inparticular, when the pixel array PA is formed as a repetitive structurein which a cell pair including the imaging cells 10 x and 10 y isrepeated, imaging cells with different spectral sensitivitycharacteristics can be uniformly arranged, and thus it is advantageousfor color resolution.

The subtraction circuit 80 may be an analog subtraction circuit, or adigital subtraction circuit. The subtraction circuit 80 may beimplemented as a micro controller including one or more memories and aprocessor, for instance. Alternatively, the subtraction circuit 80 maybe part of the horizontal signal read circuit 46 (see, for instance,FIG. 2).

As described above, according to the embodiment of the presentdisclosure, imaging cells with different spectral sensitivitycharacteristics can be mixed in the pixel array while reducing thecomplexity of circuits. FIG. 36 illustrates an exemplary configurationof an imaging device according to still another embodiment of thepresent disclosure. In contrast to the example described with referenceto FIG. 1, the pixel array PA of the imaging device 100F illustrated inFIG. 36 has multiple imaging cells 20 each including a pair of theimaging cells 20 x and 20 y. The imaging cells 20 x and 20 y is a pairof imaging cells arranged adjacent to each other in the row direction orthe column direction in the pixel array PA. The imaging region of theimaging device 100F typically includes a repetitive structure in which aunit formed by a pair of cells 20 x and 20 y is repeated. The imagingcells 20 x and 20 y may be arranged diagonally adjacent to each other inthe pixel array PA. As schematically illustrated in FIG. 36, the imagingdevice 100F has a photoelectric converter PC2 including the pixelelectrodes 61 x and 61 y, the opposite electrode 62, and a photoelectricconversion structure 66. The imaging cell 20 x has a photoelectricconverter PC2 x including an opposite electrode 62 x which is part ofthe opposite electrodes 62, a photoelectric conversion structure 66 xand the pixel electrode 61 x. Similarly, the imaging cell 20 y has aphotoelectric converter PC2 y including an opposite electrode 62 y whichis part of the opposite electrodes 62, a photoelectric conversionstructure 66 y and the pixel electrode 61 y. Similarly to theabove-described examples, the pixel electrode 61 x is connected to thegate of the signal detection transistor 21 of the charge detector CDx,and the pixel electrode 61 y is connected to the gate of the signaldetection transistor 21 of the charge detector CDy.

The photoelectric conversion structures 66 x and 66 y are differentportions in a common photoelectric conversion layer disposed overmultiple imaging cells 20. That is, unlike the examples described above,the photoelectric conversion structures 66 x and 66 y in theconfiguration illustrated to FIG. 36 do not include a multilayerstructure such as the photoelectric conversion structure 64 having thefirst photoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b. Therefore, in this example, the spectralsensitivity characteristic of the photoelectric converter PC2 x and thespectral sensitivity characteristic of the photoelectric converter PC2 yshow the same change for a change in the voltage applied across a pairof electrodes by which the photoelectric conversion structure 66 isinterposed. In other words, the wavelength range to which the imagingcell 20 x has sensitivity, and the wavelength range to which the imagingcell 20 y has sensitivity are in common. However, as described below,the sensitivity can be made different between the imaging cell 20 x andthe imaging cell 20 y by making the potential difference Φx between theopposite electrode 62 x and the pixel electrode 61 x different from thepotential difference Φy between the opposite electrode 62 y and thepixel electrode 61 y.

in a process of forming the photoelectric converter PC2, thephotoelectric conversion structures 66 x and 66 y are simultaneouslyformed by formation of the photoelectric conversion structure 66, andare positioned in the same layer in a section perpendicular to the majorsurface of the semiconductor substrate 50. The photoelectric conversionstructure 66 is typically composed of a photoelectric conversionmaterial including at least two types of materials that are a firstmaterial serving as a donor and a second material serving as anacceptor.

FIG. 37 illustrates an example of the voltage dependence of externalquantum efficiency of the photoelectric conversion structure 66. FIG. 37illustrates a result of measurement of an external quantum efficiency ofa sample for light with a wavelength of 880 nm when the potentialdifference applied across the upper-surface electrode and thelower-surface electrode is changed, the sample being produced bysequentially depositing an ITO film serving as a lower-surfaceelectrode, an electronic blocking layer, a photoelectric conversionlayer, and an Al film serving as an upper-surface electrode on a glasssubstrate by vacuum deposition. For the measurement of an externalquantum efficiency, spectral sensitivity measuring device CEP-25RRmanufactured by Bunkoukeiki Co., Ltd was used.

Here, a photoelectric conversion layer was formed by co-evaporating tinnaphthalocyanine (SnNc) and C70 so that the volume ratio between SnNcand C70 becomes 1:1. In the obtained photoelectric conversion layer,SnNc serves as electron-donating molecules, and C70 serves aselectron-accepting molecules. As the material of the electronic blockinglayer, bis (carbazolyl) benzodifuran (CZBDF) which is an ambipolarorganic semiconductor is used. The thicknesses of the lower-surfaceelectrode, the electronic blocking layer, the photoelectric conversionlayer, and the upper-surface electrode are 150 nm, 10 nm, 60 nm, and 80nm, respectively.

In FIG. 37, the horizontal axis indicates the voltage VITO applied tothe lower-surface electrode relative to the potential of theupper-surface electrode. For instance, a state where the voltage appliedto the lower-surface electrode is −2 V corresponds to a state where apositive voltage of 2V is applied to the opposite electrode 62 relativeto the potential of the pixel electrode 61 x or 61 y. Referring to theplots when a voltage of −2V is applied to the lower-surface electrode,in this example, approximately 18% of the external quantum efficiencywas obtained.

Here, when the voltage applied to the lower-surface electrode is reducedto −8V, the external quantum efficiency of 72% was obtained, and ascompared with the case where a voltage of −2 V was applied to thelower-surface electrode, the external quantum efficiency was increasedby approximately 4 times. This corresponds to an increase of sensitivityby 4 times. Since 20 log of 104=12 dB, this result demonstrates thatwhen the same configuration as in the sample of this example is appliedto the photoelectric conversion structure 66, the dynamic range can beexpanded by approximately 12 dB by increasing the potential differenceapplied across the pixel electrode and the opposite electrode from 2 Vto 8 V.

FIG. 36 is referred to again. Also in the embodiment described here,before an accumulation of signal charges is started, the bias voltageapplied to the photoelectric conversion structure are made differentbetween the imaging cell 20 x and the imaging cell 20 y adjacent to eachother. That is, at the time of start of a charge accumulation period, inother words, immediately after reset of the potential of a pixelelectrode and before start of accumulation of charges in the chargeaccumulation region, the relationship Φx≠Φy holds. For instance, whenΦx=8V and Φy=2V, it is possible to improve the sensitivity of theimaging cell 20 x, for instance, by approximately 4 times than thesensitivity of the imaging cell 20 y, thus the imaging cells 20 x and 20y can serve as a cell with high sensitivity and a cell with lowsensitivity, respectively.

A high S/N ratio is achieved by the imaging cell 20 x with relativelyhigh sensitivity even under low illumination. On the other hand, blownout highlights is prevented even under high illumination by the imagingcell 20 y with relatively low sensitivity. Therefore, high dynamic rangeimaging is possible by synthesizing image data based on the imagesignals obtained by the imaging cells 20 x and 20 y. Furthermore, inthis embodiment, the imaging cells 20 x and 20 y are arranged adjacentto each other in the pixel array PA, thus, an image signal based on thelight coming from substantially the same azimuth can be obtained by apair of imaging cells 20 x and 20 y, and synchronization between theimage signal obtained by the imaging cell 20 x and the image signalobtained by the imaging cell 20 y adjacent to the imaging cell 20 x canbe guaranteed. In other words, the pixel value of one pixel in an imageis determined by the image signal obtained by the pair of the imagingcells 20 x and 20 y adjacent to each other, and thereby a high dynamicrange image with guaranteed synchronization can be obtained.

In this manner, a photoelectric conversion layer, in which the externalquantum efficiency is changed according to a change in the bias voltageapplied, is applied to the photoelectric conversion structure, andthereby it is possible to electrically change the sensitivity withoutadding a new circuit device to the imaging cell. From FIG. 37, it isfound that in order to obtain a photoelectric conversion structurehaving such a property, it is not required to form a multilayerstructure such as the photoelectric conversion structure 64 having thefirst photoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b.

With the configuration illustrated to FIG. 36, the sensitivity betweenthese can be made different by intentionally creating a state such thatthe relationship of Φx≠Φy holds between the imaging cells 20 x and 20 yimmediately before accumulation of signal charges. Therefore, the effectof expanded dynamic range is obtained with a relatively simple circuitconfiguration, and multiple image signals obtained with changedsensitivities can be collectively obtained by the imaging device 100F.Therefore, a high dynamic range image with guaranteed synchronizationcan be formed based on, for instance, the image signal obtained from apair of imaging cells 20 x and 20 y arranged adjacent to each other.

The configuration of any one of the examples described above may beapplied to the configuration to form a state of Φx≠Φy immediately beforethe accumulation of signal charges. For instance, the circuitconfiguration described with reference to FIG. 3 may be applied to thecharge detectors CDx and CDy. The opposite electrode voltage V_(OPP) isused in common and V_(RST) 1≠V_(RST) 2 is set over the chargeaccumulation period, and thereby the bias voltage applied to thephotoelectric conversion structure before accumulation of signal chargesis started can be made different between the imaging cell 20 x and theimaging cell 20 y. The circuit configuration described with reference toFIG. 10 may be applied to the charge detectors CDx and CDy, and thereference voltage V_(REF) can be made different between the imagingcells 20 x and 20 y. Even with this configuration, the bias voltageapplied to the photoelectric conversion structure at the time of startof a charge accumulation period can be made different between theimaging cell 20 x and imaging cell 20 y.

The opposite electrode may be electrically separated between the imagingcell 20 x and the imaging cell 20 y by using the circuit configurationas illustrated in FIG. 11, and the potential of the opposite electrodeimmediately before the start of accumulation of signal charges may bemade different between the imaging cell 20 x and the imaging cell 20 y.Even with this configuration, the state of Φx≠Φy can be achieved.

Alternatively, the potential of the pixel electrode 61 x and/or thepixel electrode 61 y may be temporarily increased in a chargeaccumulation period in a frame period by using the circuit configurationillustrated in FIGS. 13, 15, and 16. With this configuration, for one ofboth of the imaging cell 20 x and the imaging cell 20 y, the biasvoltage applied to the photoelectric conversion structure 64 in a chargeaccumulation period can be selectively changed, and differentsensitivities between these cells are obtained. One of the imaging cells20 x and 20 y may serve as the imaging cell that shows a relativelylarge change in the potential difference Φ for a change in the voltageapplied to the first voltage line 31, and the other imaging cell mayserve as the imaging cell that shows almost no change in the potentialdifference Φ.

Instead of disposing a microlens independently on each of the imagingcells 20 x and 20 y arranged adjacent to each other, a microlens and/ora color filter may be shared between these cells as described withreference to FIG. 5.

It is to be noted that the above-mentioned the voltage supply circuit 41may be configured to apply a predetermined voltage to the first voltageline 31 at the time of operation of the imaging device and is notlimited to a specific power supply circuit. The voltage supply circuit41 may be a circuit that generates a predetermined voltage, or a circuitthat converts a voltage supplied from another power supply into apredetermined voltage. Similarly, the voltage supply circuit 42 may beconfigured to allow a predetermined voltage to be applied to, forinstance, the accumulation control line 35 at the time of operation ofthe imaging device. Each of the voltage supply circuits 41 and 42 may bepart of a single voltage supply circuit, or a separate independentvoltage supply circuit. It is to be noted that one or both of thevoltage supply circuits 41 and 42 may be part of the vertical scanningcircuit 48. Alternatively, a voltage from the voltage supply circuit 41and/or an opposite electrode voltage from the voltage supply circuit 42may be supplied to each imaging cell via the vertical scanning circuit48.

(Photoelectric Conversion Structure)

As described above, in Embodiments of the present disclosure, thephotoelectric conversion structure 64 in the photoelectric convertersPCx and PCy includes a multilayer structure that has the firstphotoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b. The photoelectric conversion structure 64 has athickness in a range 100 nm or greater and 1000 nm or less, forinstance. The first photoelectric conversion layer 64 a and the secondphotoelectric conversion layer 64 b include the first material and thesecond material, respectively, and in an aspect of the presentdisclosure, the first photoelectric conversion layer has a greaterimpedance than the second photoelectric conversion layer has. With thisconfiguration, it is possible to switch the spectral sensitivitycharacteristic in the photoelectric converter PC by changing the voltageapplied across the pixel electrode and the opposite electrode. It ispossible to switch the wavelength range of an obtainable image byswitching the spectral sensitivity characteristic in the imaging cell10. It is to be noted that in the present description, for the sake ofsimplicity, the term of “impedance” may be used to indicate “absolutevalue of impedance”.

In another aspect in the present disclosure, the ionization potential ofthe first material is greater than the ionization potential of thesecond material by 0.2 eV or more. As described later, when thedifference between the ionization potentials of the first materialIncluded in the first photoelectric conversion layer 64 a and the secondmaterial included in the second photoelectric conversion layer 64 b ishigh to some extent, even when the impedance difference between thefirst photoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b is small, the spectral sensitivity characteristicin the photoelectric converter PC can be changed by changing the voltageapplied across the pixel electrode and the opposite electrode.

FIG. 18 illustrates an example of a sectional structure of thephotoelectric converter. It is to be noted that the fundamentalconfiguration of the above-described photoelectric converters PCx andPCy is in common. Therefore, hereinafter, the pixel electrode 61 x ofthe photoelectric converter PCx and the pixel electrode 61 y of thephotoelectric converter PCy are simply called a pixel electrode 61without distinguishing between the pixel electrodes 61 x and 61 y.

As already described, the photoelectric converter PC includes the pixelelectrode 61, the opposite electrode 62, and the photoelectricconversion structure 64 interposed therebetween. The photoelectricconversion structure 64 typically has multiple layers including anorganic material. In the configuration illustrated to FIG. 18, thephotoelectric conversion structure 64 includes a multilayer structure ofthe first photoelectric conversion layer 64 a and the secondphotoelectric conversion layer 64 b. As illustrated, in this instance,the second photoelectric conversion layer 64 b is positioned between thefirst photoelectric conversion layer 64 a and the opposite electrode 62.

In the configuration illustrated to FIG. 18, the photoelectricconversion structure 64 includes an electronic blocking layer 64 eb anda positive hole transport layer 64 ht between the first photoelectricconversion layer 64 a and the pixel electrode 61. The electronicblocking layer 64 eb is adjacent to the pixel electrode 61, and thepositive hole transport layer 64 ht is adjacent to the firstphotoelectric conversion layer 64 a. Also, the photoelectric conversionstructure 64 includes an electron transport layer 64 et and a positivehole blocking layer 64 hb between the second photoelectric conversionlayer 64 b and the opposite electrode 62. The positive hole blockinglayer 64 hb is adjacent to the opposite electrode 62, and the electrontransport layer 64 et is adjacent to the second photoelectric conversionlayer 64 b.

The electronic blocking layer 64 eb illustrated in FIG. 18 is providedto reduce dark current caused by injection of electrons from the pixelelectrode 61, and the positive hole blocking layer 64 hb is provided toreduce dark current caused by injection of positive holes from theopposite electrode 62. It is to be noted that the electronic blockinglayer 64 eb and the positive hole blocking layer 64 hb each have afunction of selectively transporting charges, and therefore are notinsulation layers. The positive hole transport layer 64 ht and theelectron transport layer 64 et are provided to efficiently transportpositive and negative charges generated in the first photoelectricconversion layer 64 a and/or the second photoelectric conversion layer64 b to the pixel electrode 61 and the opposite electrode 62,respectively. The materials of which the electronic blocking layer 64eb, the positive hole blocking layer 64 hb, the positive hole transportlayer 64 ht, and the electron transport layer 64 et are composed can beselected from publicly known materials in consideration of bondingstrength, stability, a difference between ionization potentials, and adifference between electron affinities between each layer and adjacentlayer. The material for forming the first photoelectric conversion layer64 a or the material for forming the second photoelectric conversionlayer 64 b may be for at least one of the materials of which theelectronic blocking layer 64 eb, the positive hole blocking layer 64 hb,the positive hole transport layer 64 ht, and the electron transportlayer 64 et are composed.

The first photoelectric conversion layer 64 a and the secondphotoelectric conversion layer 64 b include the first material and thesecond material, respectively. Therefore, typically, impedance per unitthickness in the first photoelectric conversion layer 64 a is differentfrom impedance per unit thickness in the second photoelectric conversionlayer 64 b. The first material and the second material are typicallysemiconductor materials. In an aspect of the present disclosure,impedance per unit thickness in the first photoelectric conversion layer64 a is greater than impedance per unit thickness in the secondphotoelectric conversion layer 64 b. The impedance depends on thethickness of a photoelectric conversion layer, and when a photoelectricconversion layer includes multiple materials, the impedance also dependson the volume ratio of those materials in the photoelectric conversionlayer. In Embodiments of the present disclosure, a layer having a highimpedance among multiple photoelectric conversion layers included in themultilayer structure can be used as the first photoelectric conversionlayer 64 a.

(Switching of Spectral Sensitivity Characteristic by Switching BiasVoltage Utilizing Impedance Difference)

When the photoelectric conversion structure 64 includes a multilayerstructure having the first photoelectric conversion layer and the secondphotoelectric conversion layer which have different impedances,application of a bias voltage between the pixel electrode 61 and theopposite electrode 62 causes a voltage proportional to impedance to beapplied to the first photoelectric conversion layer and the secondphotoelectric conversion layer. In other words, an electric field with amagnitude proportional to impedance is applied to the firstphotoelectric conversion layer and the second photoelectric conversionlayer. An external quantum efficiency (E.Q.E.) for a wavelength rangecan be changed by changing the potential difference Φ to be appliedacross the pixel electrode 61 and the opposite electrode 62 thatinterpose a photoelectric conversion structure including a multilayerstructure having photoelectric conversion layers with differentimpedances. In other words, in the imaging cell 10 having such aphotoelectric conversion structure in the photoelectric converter PC,the spectral sensitivity characteristic may be electrically changed. Forinstance, when the potential difference to be applied is changed frompotential difference Φ1 to the potential difference Φ2, an increase inthe E.Q.E. in the absorption peak wavelength of the second material isgreater than an increase in the E.Q.E. in the absorption peak wavelengthof the first material.

For instance, let Z1 and Z2 be the impedance of the first photoelectricconversion layer 64 a, and the impedance of the second photoelectricconversion layer 64 b, respectively, then when Z1>Z2, a greater voltageis applied to the first photoelectric conversion layer 64 a as comparedwith the second photoelectric conversion layer 64 b. Therefore, evenwhen the bias between the pixel electrode 61 and the opposite electrode62 is small, an electric field having a sufficient magnitude for movingthe charges generated by the photoelectric conversion to an electrodecan be applied to the first photoelectric conversion layer 64 a. Inother words, positive and negative charges generated by thephotoelectric conversion can reach the pixel electrode 61 and theopposite electrode 62, respectively. Specifically, signal chargesgenerated by irradiation of the first photoelectric conversion layer 64a with light are collected by the pixel electrode 61, and areaccumulated in the charge accumulation region.

On the other hand, the electric field applied to the secondphotoelectric conversion layer 64 b is smaller than the electric fieldapplied to the first photoelectric conversion layer 64 a. Thus, when asmaller potential difference is applied to the photoelectric conversionstructure 64 between the pixel electrode 61 and the opposite electrode62, the electric field applied to the second photoelectric conversionlayer 64 b may fall below a necessary magnitude for signal charges toreach the pixel electrode 61, the signal charges being generated byirradiation of the second photoelectric conversion layer 64 b withlight. If signal charges do not reach the pixel electrode 61, even whensignal charges are generated in the second photoelectric conversionlayer 64 b, the signal charges are not accumulated in the chargeaccumulation region. Therefore, the imaging cell 10 does not havesufficient sensitivity to the light with a wavelength rangecorresponding to the absorption spectrum of the material of which thesecond photoelectric conversion layer 64 b is composed, particularly theabsorption spectrum of the second material.

When the voltage applied across the opposite electrode 62 and the pixelelectrode 61 is increased, the voltage applied to the secondphotoelectric conversion layer 64 b is also increased. That is, theelectric field applied to the second photoelectric conversion layer 64 bis increased, and the signal charges reach the pixel electrode 61, forinstance, by supplying a voltage with a larger absolute value to pixelelectrode 61 or the opposite electrode 62. Therefore, the imaging cell10 has sensitivity to the light with a wavelength range corresponding tothe absorption spectrum of the material (particularly, the secondmaterial) of which the second photoelectric conversion layer 64 b, inaddition to the light with a wavelength range corresponding to theabsorption spectrum of the material (particularly, the first material)of which the first photoelectric conversion layer 64 a is composed.

In this manner, a multilayer structure having the first photoelectricconversion layer, and the second photoelectric conversion layer whichhas impedance smaller than the impedance of the first photoelectricconversion layer is applied, and thus the spectral sensitivitycharacteristic can be switched by switching a voltage supplied to thepixel electrode 61 or the opposite electrode 62. The ratio of theimpedance of the first photoelectric conversion layer 64 a to theimpedance of the second photoelectric conversion layer 64 b is typicallyin a range of 100 times or more and 10¹⁰ times or less. When the ratioof the impedance of the first photoelectric conversion layer 64 a to theimpedance of the second photoelectric conversion layer 64 b exceeds atleast 44 times, such switching of the spectral sensitivitycharacteristic by switching the bias voltage can be achieved.

As the combination of the first material and the second material, forinstance, a combination of a material exhibiting a high absorptioncoefficient in visible range and a material exhibiting a high absorptioncoefficient in infrared range may be used. With this combination ofmaterials, it is possible to provide an imaging device that can obtaininformation on one or both of illumination of visible light andillumination of infrared light.

Typically, the first photoelectric conversion layer 64 a and the secondphotoelectric conversion layer 64 b include electron-donating (or donorproperty, p-type) molecules, and electron-accepting (or acceptorproperty, n-type) molecules.

For instance, electron-donating molecules are used as the first materialIncluded in the first photoelectric conversion layer 64 a and the secondmaterial included in the second photoelectric conversion layer 64 b. Atypical instance of electron-donating molecules is an organic p-typesemiconductor, and is mainly represented by a positive hole transportorganic compound. The electron-donating molecules have a property ofbeing likely to donate electrons. Examples of an organic p-typesemiconductor include triaryl amine compounds such as DTDCTB, benzidinecompounds, pyrazoline compounds, styryl amine compounds, hydrazonecompounds, triphenylmethane compounds, carbazole compounds, polysilanecompounds, α-sexithiophene (hereinafter referred to as “α-6T”),thiophene compounds such as P3HT, phthalocyanine compounds, cyaninecompounds, merocyanine compounds, oxonol compounds, polyamine compounds,indolic compounds, pyrrole compounds, pyrazole compounds, polyarylenecompounds, condensed aromatic carbon ring compounds, (naphthalenederivatives, anthracene derivatives, phenanthrene derivatives, tetracenederivatives such as rubrene, pyrene derivatives, perylene derivatives,fluoranthene derivatives), and metal complexes having anitrogen-containing hetero ring compound as a ligand. Examples ofphthalocyanine compounds include copper phthalocyanine (CuPc),subphthalocyanine (SubPc), aluminium chloride phthalocyanine (ClAlPc),Si(OSiR3)₂Nc (R indicates alkyl with a carbon number of 1 to 18), turnipphthalocyanine (SnNc), and lead phthalocyanine (PbPc). Donor organicsemiconductors are not limited to these, and organic compounds with anionization potential lower than the ionization potential of organiccompounds used as n-type (acceptor property) compounds can be used asdonor organic semiconductors. The ionization potential is the differencebetween the vacuum level and the energy level of a highest occupiedmolecular orbital (HOMO).

A typical instance of electron-accepting molecules is an organic n-typesemiconductor, and is mainly represented by an electron transportorganic compound. The electron-accepting molecules have a property ofbeing likely to accept electrons. Examples of an organic n-typesemiconductor include fullerene such as C₆₀ and C₇₀, fullerenederivatives such as phenyl C₆₁ butyric-acid methyl ester (PCBM),condensed aromatic carbon ring compounds, (naphthalene derivatives,anthracene derivatives, phenanthrene derivatives, tetracene derivatives,pyrene derivatives, perylene derivatives, fluoranthene derivatives), 5to 7 membered hetero ring compounds containing nitrogen atoms, oxygenatoms, and sulfur atoms (for instance, pyridine, pyrazine, pyrimidine,pyridazine, triazine, quinoline, quinoxaline, quinazoline, phthalazine,cinnoline, isoquinoline, pteridine, acridine, phenazine, phenanthroline,tetrazole, pyrazole, imidazole, thiazole, oxazole, indazole,benzimidazole, benzotriazole, benzoxazole, benzothiazole, carbazole,purine, triazolo-pyridazine, triazolo-pyrimidine, tetrazaindene,oxadiazole, imidazopyridine, pyrrolidine, pyrrolopyridine,thiadiazolopyridine, dibenzazepine, tribenzazepine), subphthalocyanine(SubPc), polyarylene compounds fluorene compounds, cyclopentadienecompounds, silyl compounds, perylene tetracarboxylic diimide compounds(PTCDI), and metal complexes having a nitrogen-containing hetero ringcompound as a ligand. Acceptor organic semiconductors are not limited tothese, and organic compounds with an electron affinity greater than theelectron affinity of organic compounds used as p-type (donor property)compounds can be used as acceptor organic semiconductors. The electronaffinity is the difference between the vacuum level and the energy levelof a lowest unoccupied molecular orbital (LUMO).

FIG. 19 illustrates the chemical formulas of SnNc, DTDCTB, and C₇₀.Without being limited to those mentioned above, as long as organiccompounds or organic molecules allows film formation by one of a drysystem and a wet system, the organic compounds and organic molecules canbe used as the material of which the first photoelectric conversionlayer 64 a is composed or the material of which the second photoelectricconversion layer 64 b is composed, regardless of low-molecular orhigh-molecular compounds.

A photoelectric conversion structure 64 having sensitivity of a desiredwavelength range can be implemented by using appropriate materials asthe first material and the second material according to a wavelengthrange to be detected. For instance, a material having a high absorptioncoefficient in the visible range and a material having a high absorptioncoefficient in the infrared range may be used as the first materialIncluded in the first photoelectric conversion layer 64 a and the secondmaterial included in the second photoelectric conversion layer 64 b,respectively. The above-mentioned DTDCTB has an absorption peak at awavelength of approximately 700 nm, and CuPc and SubPc have absorptionpeaks at wavelengths of approximately 620 nm and 580 nm, respectively.Rubrene has an absorption peak at a wavelength of approximately 530 nm,and α-6T has an absorption peak at a wavelength of approximately 440 nm.In short, the absorption peaks of these materials are in the wavelengthrange of visible light, and can be used as the first material, forinstance. In contrast, SnNc has an absorption peak at a wavelength ofapproximately 870 nm, and ClAlPc has an absorption peak at a wavelengthof approximately 750 nm. In short, the absorption peaks of thesematerials are in the wavelength range of infrared light, and can be usedas the second material, for instance.

The second material included in the second photoelectric conversionlayer 64 b includes SnNc represented by the following structural formula(1), for instance.

R¹ to R²⁴ in the structural formula (1) each independently indicate ahydrogen atom or a substituent. The substituent is not limited to aspecific substituent. The substituent may be a deuterium atom, a halogenatom, alkyl groups (including a cycloalkyl group, a bicycloalkyl group,a tricycloalkyl group), an alkenyl group (including a cycloalkenylgroup, a bicycloalkenyl group), an alkynyl group, an aryl group, aheterocyclic group (may be called a heterocyclic group), a cyano group,a hydroxy group, a nitro group, a carboxy group, an alkoxy group, anaryloxy group, a silyloxy group, a heterocyclic oxy group, a acyloxygroup, a carbamoyloxy group, an alkoxy carbonyloxy group, an aryloxycarbonyloxy group, an amino group (including an anilino group), anammonio group, an acylamino group, an aminocarbonyl amino group, analkoxycarbonylamino group, an aryloxycarbonylamine group, anaryloxycarbonylamine group, a sulfamoylamino group, analkylsulfonylamino group, arylsulfonylamino group, an mercapto group, analkylthio group, an arylthio group, a heterocyclic thio group, asulfamoyl group, a sulfonic group, an alkylsulfinyl group, anarylsulfinyl group, an alkylsulfonyl group, an arylsulfonyl group, anacyl group, an aryloxycarbonyl group, an alkoxycarbonyl group, acarbamoyl group, an arylazo group, a heterocyclic azo group, an imidegroup, a phosphino group, a phosphinyl group, a phosphinyloxy group, aphosphinyl amino group, a phosphono group, a silyl group, a hydrazinogroup, a ureido group, a boron acid group (—B(OH)₂), a phosphato group(—OPO(OH)₂), a sulphato group (—OSO₃H), or other publicly knownsubstituent groups.

Commercially available products may be used as SnNc represented by theabove-mentioned structural formula (1). Alternatively, for instance, asdescribed in Japanese Unexamined Patent Application Publication No.2010-232410, SnNc represented by the above-mentioned structural formula(1) can be synthesized using naphthalene derivative represented by thestructural formula (2) below as starting material. R²⁵ to R³⁰ in thestructural formula (2) can be the same substituent groups as R¹ to R²⁴in the structural formula (1).

From the viewpoint of easiness of control over aggregation state ofmolecules in tin naphthalocyanine represented by the above-mentionedstructural formula (1), it is useful when eight or more of R¹ to R²⁴ areeach a hydrogen atom or a deuterium atom, and it is more useful when 16or more of R¹ to R²⁴ are each a hydrogen atom or a deuterium, and it isfurther useful when all of R¹ to R²⁴ are each a hydrogen atom or adeuterium. In addition, SnNc represented by the following structuralformula (3) is advantageous from the viewpoint of easiness of synthesis.

SnNc represented by the above-mentioned structural formula (1) has anabsorption peak in a wavelength range of approximately 200 nm or moreand 1100 nm or less. For instance, SnNc represented by theabove-mentioned structural formula (3) has an absorption peak at awavelength of approximately 870 nm as illustrated in FIG. 23. FIG. 23 isan instance of an absorption spectrum in a photoelectric conversionlayer including SnNc represented by the above-mentioned structuralformula (3). It is to be noted that for the measurement of an absorptionspectrum, a sample was used in which a photoelectric conversion layerwith a thickness of 30 nm is stacked on a quartz substrate.

For instance, a material having an absorption peak in a first wavelengthrange included in the visible range is used as the first material, and amaterial having an absorption peak in a second wavelength range includedin the infrared range is used as the second material, thereby making itpossible to electrically change the sensitivity in the infrared range.Needless to say, a material having a high absorption coefficient in theinfrared range and a material having a high absorption coefficient inthe visible range may be used as the first material and the secondmaterial, respectively.

For instance, it is assumed that impedance Z1 of the first photoelectricconversion layer for which a material having a high absorptioncoefficient in the visible light is used as the first material isgreater than the impedance Z2 of the second photoelectric conversionlayer for which a material having a high absorption coefficient in theinfrared light is used as the second material (Z1>Z2). At this point,when the voltage applied across the opposite electrode 62 and the pixelelectrode 61 is lower than or equal to a threshold value, thephotoelectric converter PC has a relatively high sensitivity in thevisible range. Therefore, an image signal based on the visible light canbe obtained. On the other hand, when the voltage applied across theopposite electrode 62 and the pixel electrode 61 is higher than athreshold value, the photoelectric converter PC has sensitivity in thevisible range and the infrared range. Therefore, an image signal basedon the visible light and the infrared light can be obtained. In otherwords, let Φ1 be a voltage that allows imaging with the visible light,and let Φ2 be a voltage that allows imaging with the visible light andthe infrared light in the potential difference applied across theopposite electrode 62 and the pixel electrode 61, then the relationshipof Φ1<Φ2 holds.

Conversely, when the impedance Z1 of the first photoelectric conversionlayer is lower than the impedance Z2 of the second photoelectricconversion layer (Z1<Z2), and the voltage applied across the oppositeelectrode 62 and the pixel electrode 61 is lower than or equal to athreshold value, the photoelectric converter PC has a relatively highsensitivity in the infrared range. Consequently, the imaging deviceaccording to an Embodiment of the present disclosure can obtain an imagesignal based on the infrared light. On the other hand, when the voltageapplied across the opposite electrode 62 and pixel electrode 61 higherthan a threshold value, the photoelectric converter PC has sensitivityin the visible light range and the infrared light range. Therefore, animage signal based on the visible light and the infrared light can beobtained. At this point, let Φ3 be a potential difference that allowsimaging with the infrared light, and let Φ4 be a potential differencethat allows imaging with the visible light and the infrared light in thevoltage applied across the opposite electrode 62 and the pixel electrode61, then the relationship of Φ3<Φ4 holds after all. What is noteworthyhere is that the wavelength range of an obtainable image can be switchedby the potential difference applied across the opposite electrode 62 andthe pixel electrode 61.

When the first photoelectric conversion layer 64 a and the secondphotoelectric conversion layer 64 b do not sufficiently have desiredsensitivity characteristic by using a single organic material, one orboth of the first photoelectric conversion layer 64 a and the secondphotoelectric conversion layer 64 b may be formed by mixing two or moreorganic materials. Alternatively, one or both of the first photoelectricconversion layer 64 a and the second photoelectric conversion layer 64 bmay be formed by stacking two or more layers including different organicmaterials. The first photoelectric conversion layer 64 a and/or thesecond photoelectric conversion layer 64 b may be, for instance, a bulkheterojunction structure layer including a p-type semiconductor and ann-type semiconductor. The bulk heterojunction structure is described indetail in Japanese Unexamined Patent Application Publication No.5553727. The entire contents of Japanese Unexamined Patent ApplicationPublication No. 5553727 are incorporated herein by reference.

FIG. 20 illustrates another instance of the sectional structure of thephotoelectric converter PC. The photoelectric conversion structure 64Aillustrated in FIG. 20 includes a multilayer structure having the firstphotoelectric conversion layer 64 a, a mixed layer 64 m, and the secondphotoelectric conversion layer 64 b. The mixed layer 64 m is a layerthat includes the first material and the second material at least, andis positioned between the first photoelectric conversion layer 64 a andthe second photoelectric conversion layer 64 b. It is to be noted thatFIG. 20 and the aforementioned FIG. 18 are merely schematic diagrams,and the boundary of each layer included in the photoelectric conversionstructure may not be strictly defined. The same goes with othersectional views of the present disclosure.

As just described, the configuration of the photoelectric converter PCis not limited to the configuration illustrated in FIG. 18. Forinstance, the arrangement of the first photoelectric conversion layer 64a and the second photoelectric conversion layer 64 b and the arrangementillustrated in FIGS. 18 and 20 may be reversed. When negative charges(typically, electrons) among positive and negative charges generated inthe photoelectric conversion structure 64 are used as signal charges, apositive hole blocking layer and an electron transport layer may be usedinstead of the electronic blocking layer 64 eb and the positive holetransport layer 64 ht, and a positive hole transport layer and anelectronic blocking layer may be used instead of the electron transportlayer 64 et and the positive hole blocking layer 64 hb.

The material of which the first photoelectric conversion layer 64 a andthe second photoelectric conversion layer 64 b are composed is notlimited to an organic semiconductor material, and the firstphotoelectric conversion layer 64 a and/or the second photoelectricconversion layer 64 b may include a compound semiconductor representedby hydrogenation amorphous silicon, CdSe, and an inorganic semiconductormaterial of a metal oxide semiconductor such as ZnO. For instance, thevolume resistivity of amorphous silicon is adjustable by changing animpurity density. The first photoelectric conversion layer 64 a and/orthe second photoelectric conversion layer 64 b may include a layercomposed of an organic material and a layer composed of an inorganicmaterial.

(Switching of Spectral Sensitivity Characteristic by Switching BiasVoltage Utilizing Ionization Potential Difference)

As described below, even when the impedance difference between the firstphotoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b is small, when the difference between theionization potentials of the first material Included in the firstphotoelectric conversion layer 64 a and the second material included inthe second photoelectric conversion layer 64 b is high to some extent,the spectral sensitivity characteristic can be changed by changing thepotential difference Φ between the pixel electrode 61 and the oppositeelectrode 62.

FIG. 21 is an energy diagram in still another configuration instance ofthe photoelectric converter PC. The rectangles in FIG. 21, schematicallyillustrate LUMO and HOMO in each material. The numerical value givennear each of the upper sides and the lower sides of these rectanglesindicates the electron affinity and the ionization potential of eachmaterial. The thick horizontal lines in FIG. 21 schematically indicatethe exemplary Fermi levels of the opposite electrode 62 and the pixelelectrode 61.

In the configuration illustrated to FIG. 21, the photoelectricconversion structure 64B has a multilayer structure in which theelectronic blocking layer 64 eb, the first photoelectric conversionlayer 64 a, and the second photoelectric conversion layer 64 b arestacked from the pixel electrode 61 to the opposite electrode 62. Inthis instance, as the first material, the second material, and thematerial for the electronic blocking layer 64 eb, rubrene, SnNc, and bis(carbazolyl) benzodifuran (CZBDF) which is an ambipolar organicsemiconductor are used, respectively. FIG. 22 illustrates the chemicalformula of CZBDF. AS schematically illustrated in FIG. 21, the firstphotoelectric conversion layer 64 a and the second photoelectricconversion layer 64 b include C₇₀ as an acceptor organic semiconductor.The first photoelectric conversion layer 64 a in this instance receivesthe visible light to generate charge pairs by photoelectric conversion,and the second photoelectric conversion layer 64 b receives the infraredlight to generate charge pairs by photoelectric conversion. Each opencircle “◯” and solid circle “●” in FIG. 21 respectively indicate apositive charge and a negative charge generated by photoelectricconversion.

As already described, when positive charges are collected by the pixelelectrode 61, a predetermined voltage is supplied to the oppositeelectrode 62, for instance, and the opposite electrode 62 has apotential higher than the potential of the pixel electrode 61. In thisstate, when the visible light is incident to the first photoelectricconversion layer 64 a and positive and negative charges are generated inthe first photoelectric conversion layer 64 a, the positive charges arecollected by the pixel electrode 61. Specifically, the imaging cell 10has sensitivity to the wavelength range of visible light with signalcharges generated by irradiation with visible light accumulated in acharge accumulation region. The negative charges transfer from the LUMOlevel to the LUMO level of C₇₀, and moves toward the opposite electrode62 by the electric field between the pixel electrode 61 and the oppositeelectrode 62. Since C₇₀ is used in common as an acceptor organicsemiconductor between the first photoelectric conversion layer 64 a andthe second photoelectric conversion layer 64 b, the negative charges,which have transferred to the LUMO level of C₇₀, continuously move tothe opposite electrode 62, and can be collected by the oppositeelectrode 62.

Here, a state is assumed in which infrared light is incident to thesecond photoelectric conversion layer 64 b, and positive and negativecharges are generated in the second photoelectric conversion layer 64 b.When attention is focused on the positive charges, the positive chargesmove toward the pixel electrode 61 by the electric field between thepixel electrode 61 and the opposite electrode 62. However, asillustrated in FIG. 21, the ionization potential of rubrene is greaterthan the ionization potential of SnNc, and thus a potential barrier forthe positive charges is formed between the HOMO level of SnNc and theHOMO level of rubrene. Therefore, when the bias between the pixelelectrode 61 and the opposite electrode 62 is low, the positive chargescannot overcome the potential barrier, and do not reach the pixelelectrode 61. This indicates a state in which the imaging cell 10 has nosensitivity to the wavelength range of infrared light.

When the bias between the pixel electrode 61 and the opposite electrode62 is increased, and energy for overcoming the potential barrier isgiven to the positive charges, the positive charges overcome thepotential barrier, and reach the pixel electrode 61. That is, thepositive charges generated in the second photoelectric conversion layer64 b can be collected by the pixel electrode 61 by applying a greaterpotential difference between the pixel electrode 61 and the oppositeelectrode 62. In other words, sensitivity in the wavelength range ofinfrared light can be given to the imaging cell 10 by switching thepotential difference Φ to be applied across the pixel electrode 61 andthe opposite electrode 62. At this point, the imaging cell 10 hassensitivity in the wavelength ranges of visible light and infraredlight, for instance.

When the difference φ from the ionization potential of the secondmaterial Included in the second photoelectric conversion layer 64 b tothe ionization potential of the first material Included in the firstphotoelectric conversion layer 64 a is approximately 0.2 eV or more, theeffect of such switching of spectral sensitivity characteristic byswitching the potential difference Φ is obtained. At this point, asillustrated in FIG. 21, in a configuration in which the secondphotoelectric conversion layer 64 b is positioned between the firstphotoelectric conversion layer 64 a and the opposite electrode 62, it isonly necessary to cause the opposite electrode 62 to be higher inpotential than the pixel electrode 61.

Like this, when the ionization potential of the first material isgreater than the ionization potential of the second material by acertain difference or more, the spectral sensitivity characteristic inthe imaging cell 10 can be electrically switched even when the impedancedifference between the first photoelectric conversion layer 64 a and thesecond photoelectric conversion layer 64 b is small. An impedancedifference may be further provided between the first photoelectricconversion layer 64 a and the second photoelectric conversion layers 64b such that the impedance difference is sufficiently large to allow thespectral sensitivity characteristic to be electrically switched.

A HOMO level of organic material can be determined, for instance, basedon photoelectron spectroscopy, and photoemission yield spectroscopy.Also, a LUMO level may be determined based on inverse photoemissionspectroscopy or by subtracting the energy at an absorption spectrum endfrom the HOMO level. [Examples]

A sample having a multilayer structure similar to an instance of theabove-described photoelectric converter PC was produced, and the changeof spectral sensitivity characteristic with respect to the change of thebias in the produced sample was evaluated by measuring E.Q.E. with thebias changed. The sample was produced in the following manner.

Example 1-1

First, a glass substrate was prepared. Subsequently, the materialslisted in Table 1 were deposited sequentially on a glass substrate byvacuum deposition, and thus a multilayer structure is formed on theglass substrate, the multilayer structure including a lower-surfaceelectrode, an electronic blocking layer, a lower-side photoelectricconversion layer, an upper-side photoelectric conversion layer, and anupper-surface electrode. Table 1 also illustrates the thickness of eachlayer formed. In the formation of the lower-side photoelectricconversion layer, SnNc and C₇₀ were co-evaporated. Similarly, theupper-side photoelectric conversion layer was formed by co-evaporatingDTDCTB and C₇₀. In the formation of the lower-side photoelectricconversion layer and the formation of the upper-side photoelectricconversion layer, the conditions for vapor deposition were adjusted sothat the volume ratio between SnNc and C₇₀, and the volume ratio betweenDTDCTB and C₇₀ become 1:1. In this manner, the sample of Example 1-1 wasobtained.

TABLE 1 THICK- LAYER MATERIAL NESS (nm) UPPER-SURFACE ELECTRODE Al 80UPPER-SIDE PHOTOELECTRIC DTDCTB:C₇₀ (1:1) 60 CONVERSION LAYER LOWER-SIDEPHOTOELECTRIC SnNc:C₇₀ (1:1) 60 CONVERSION LAYER ELECTRON BLOCKING LAYERCZBDF 10 LOWER-SURFACE ELECTRODE ITO 150

Next, spectral sensitivity measuring device CEP-25RR manufactured byBunkoukeiki Co., Ltd was connected to the lower-surface electrode andthe upper-surface electrode, and E.Q.E. in the sample of Example 1-1 wasmeasured while changing the bias to be applied across the lower-surfaceelectrode and the upper-surface electrode. Here, with the quantity oflight to a measurement target fixed, E.Q.E. was measured by changing thepotential of the lower-surface electrode to −3 V, −5 V, −8 V, −10 V, and−11 V with the potential of the upper-surface electrode is grounded.Application of these biases corresponds to the configuration in whichpositive charges are collected by the pixel electrode 61 in theabove-described photoelectric converter PC. Specifically, in thisinstance, positive charges generated by photoelectric conversion movetoward the lower-surface electrode, and the lower-surface electrode andthe upper-surface electrode in the sample of Example 1-1 can beassociated with the pixel electrode 61 and the opposite electrode 62 inthe above-described photoelectric converter PC, respectively. However,because light was incident from the glass substrate side in themeasurement, ITO was used as the material for the lower-surfaceelectrode, and Al was used as the material for the upper-surfaceelectrode.

FIG. 24 illustrates the voltage dependence of E.Q.E. in the sample ofExample 1-1. Each of the graphs illustrated in FIG. 24 is normalized sothat the peak value of E.Q.E. equals 1. It is to be noted that in eachgraph after FIG. 24 related to the voltage dependence of E.Q.E., isnormalized so that the peak value of E.Q.E. equals 1.

When the absolute value of the bias voltage applied to the lower-surfaceelectrode is small, in other words, when the potential differenceapplied across two electrodes is small, it is found from FIG. 24 thatE.Q.E. in near an absorption peak position of SnNc included in thelower-side photoelectric conversion layer has a relatively small value.In short, the sensitivity in the infrared range is low. In contrast, inthe visible range where DTDCTB included in the upper-side photoelectricconversion layer has an absorption peak, a relatively high E.Q.E. wasobtained. Furthermore, it is found from FIG. 24 that when the absolutevalue of the bias voltage applied to the lower-surface electrode isincreased, the E.Q.E. in the infrared range increases as the absolutevalue of the bias voltage is increased. Consequently, it is found thatsensitivity in the wavelength range corresponding to the absorptionspectrum of SnNc increases depending on the magnitude of the bias.

For instance, at the wavelength (near 870 nm) corresponding to theabsorption peak of SnNc, the E.Q.E. when the potential of thelower-surface electrode was set to −3 V is compared with the E.Q.E. whenthe potential of the lower-surface electrode was set to −11 V, thelatter was approximately 33.7 times the former. It is although it didnot illustrate in FIG. 24, at the wavelength (near 870 nm) correspondingto the absorption peak of SnNc, the E.Q.E. when the potential of thelower-surface electrode was set to −15 V was approximately 33.7 timesthe E.Q.E. when the potential of the lower-surface electrode was set to−3 V.

Next, the impedance of the upper-side photoelectric conversion layer wascompared with the impedance of the lower-side photoelectric conversionlayer. For the measurement of impedance, a sample having only theupper-side photoelectric conversion layer between the lower-surfaceelectrode and the upper-surface electrode, and a sample having only thelower-side photoelectric conversion layer between the lower-surfaceelectrode and the upper-surface electrode were used. The configurationof the sample used for measurement of the impedance of the upper-sidephotoelectric conversion layer is the same as the configuration of thesample of Example 1-1 except that the lower-side photoelectricconversion layer and the electronic blocking layer were not formed, andthe thickness of the upper-side photoelectric conversion layer was 200nm. The configuration of the sample used for measurement of theimpedance of the lower-side photoelectric conversion layer is the sameas the configuration of the sample of Example 1-1 except that theupper-side photoelectric conversion layer and the electronic blockinglayer were not formed, and the thickness of the lower-side photoelectricconversion layer was 200 nm. For the measurement and analysis of theimpedance, ModuLab XM ECS manufactured by TOYO Corporation and Zplotsoftware were used. Frequency sweep mode was used as the operation mode,the amplitude was set to 10 mV, and the frequency was changed from 1 Hzto 1 MHz. Measurement was made with start delay of 5 sec. The values ofimpedances were compared between the upper-side photoelectric conversionlayer and the lower-side photoelectric conversion layer with the biasvoltage to the lower-surface electrode with respect to the upper-surfaceelectrode at −8 V and the frequency at 1 Hz.

The value of impedance with the bias voltage of −8 V and the frequencyof 1 Hz was 7.5×10⁶Ω for the upper-side photoelectric conversion layerincluding DTDCTB, and 4.2×10³Ω for the lower-side photoelectricconversion layer including SnNc. That is, the impedance of theupper-side photoelectric conversion layer was approximately 1800 timesgreater than the impedance of the lower-side photoelectric conversionlayer.

FIG. 25 illustrates the relationship between E.Q.E. and applied electricfield with wavelengths of 460 nm, 540 nm, 680 nm, and 880 nm for thesample of Example 1-1. The horizontal axis of the graph illustrated inFIG. 25 indicates the value obtained by dividing the bias voltageapplied across the upper-surface electrode and the lower-surfaceelectrode by the sum of thicknesses of the upper-side photoelectricconversion layer, the lower-side photoelectric conversion layer, and theelectronic blocking layer. That is, the horizontal axis of the graphillustrated in FIG. 25 corresponds to the magnitude of the electricfield applied across the upper-surface electrode and the lower-surfaceelectrode.

In the instance illustrated in FIG. 25, the E.Q.E. for the light with awavelength of 880 nm is substantially zero with the electric fieldstrength less than approximately 4×10⁵ V/cm, and the E.Q.E. starts toincrease with the electric field strength of a threshold value orgreater, here approximately 4×10⁵ V/cm or greater. A sufficiently highbias can be applied to a layer having a relatively lower impedance oftwo photoelectric conversion layers by applying a sufficiently high biasto the photoelectric conversion structure (for instance, see FIG. 18)including a multilayer structure having the first and secondphotoelectric conversion layers. From FIG. 25, it is found that when asufficiently high bias is applied to a layer (that is, here thelower-side photoelectric conversion layer) having a relatively lowerimpedance between two photoelectric conversion layers, the E.Q.E. of thelayer has a relatively large value. From FIG. 25, it is found that theE.Q.E. for each of wavelengths 460 nm, 540 nm, 680 nm, and 880 nm tendsto be saturated when the magnitude of the electric field between theupper-surface electrode and the lower-surface electrode is approximately9×10⁵ V/cm or greater.

Reference Example 1

A sample of Reference Example 1 substantially the same as the sample ofExample 1-1 was produced except that a mixed layer including SnNc andDTDCTB was disposed between the lower-side photoelectric conversionlayer and the upper-side photoelectric conversion layers. Table 2 belowlists the material and the thickness of each layer in the sample ofReference Example 1. The mixed layer was formed by co-evaporating threematerials: SnNc, DTDCTB, and C₇₀. In the formation of the mixed layer,the conditions for vapor deposition were adjusted so that the volumeratio between SnNc, DTDCTB, and C₇₀ becomes 1:1:8. Also, in theformation of the lower-side photoelectric conversion layer and theformation of the upper-side photoelectric conversion layer, theconditions for vapor deposition were adjusted so that the volume ratiobetween SnNc and C₇₀, and the volume ratio between DTDCTB and C₇₀ become1:4.

TABLE 2 THICK- LAYER MATERIAL NESS (nm) UPPER-SURFACE ELECTRODE Al 80UPPER-SIDE PHOTOELECTRIC DTDCTB:C₇₀ (1:4) 50 CONVERSION LAYER MIXEDLAYER SnNc:DTDCTB:C₇₀ 20 (1:1:8) LOWER-SIDE PHOTOELECTRIC SnNc:C₇₀ (1:4)50 CONVERSION LAYER ELECTRON BLOCKING LAYER CZBDF 10 LOWER-SURFACEELECTRODE ITO 150

Similarly to the sample of Example 1-1, for the sample of ReferenceExample 1, the voltage dependence of the E.Q.E. was measured. FIG. 26illustrates the voltage dependence of E.Q.E. in the sample of ReferenceExample 1.

As illustrated in FIG. 26, similarly to the sample of Example 1-1, inthe sample of Reference Example 1, due to the increase in the absolutevalue of the bias voltage applied to the lower-surface electrode, theE.Q.E. at near (near 870 nm) the absorption peak position of SnNcincluded in the lower-side photoelectric conversion layer increases.From FIG. 26, even with the configuration in which the mixed layerincluding both the first material and the second material is disposedbetween the photoelectric conversion layers in the multilayer structurehaving the first and second photoelectric conversion layers, the effectof sensitivity modulation can be obtained by switching the bias voltage.

Example 1-2

Similarly to the sample of Example 1-1, a sample of Example 1-2 wasproduced except that ClAlPc and C₇₀ were used as the material to formthe lower-side photoelectric conversion layer. In the formation of thelower-side photoelectric conversion layer, the conditions for vapordeposition were adjusted so that the volume ratio between ClAlPc and C₇₀becomes 1:9. Table 3 below lists the material and the thickness of eachlayer in the sample of Example 1-2.

TABLE 3 THICK- LAYER MATERIAL NESS (nm) UPPER-SURFACE ELECTRODE Al 80UPPER-SIDE PHOTOELECTRIC DTDCTB:C₇₀ (1:9) 60 CONVERSION LAYER LOWER-SIDEPHOTOELECTRIC ClAlPc:C₇₀ (1:1) 60 CONVERSION LAYER ELECTRON BLOCKINGLAYER CZBDF 10 LOWER-SURFACE ELECTRODE ITO 150

Comparative Example 1

Similarly to the sample of Example 1-2, a sample of Comparative Example1 was produced except that the conditions for vapor deposition wereadjusted so that the volume ratio between ClAlPc and C₇₀, and the volumeratio between DTDCTB and C₇₀ become 1:4. Table 4 below lists thematerial and the thickness of each layer in the sample of ComparativeExample 1.

TABLE 4 THICK- LAYER MATERIAL NESS (nm) UPPER-SURFACE ELECTRODE Al 80UPPER-SIDE PHOTOELECTRIC DTDCTB:C₇₀ (1:4) 60 CONVERSION LAYER LOWER-SIDEPHOTOELECTRIC ClAlPc:C₇₀ (1:4) 60 CONVERSION LAYER ELECTRON BLOCKINGLAYER CZBDF 10 LOWER-SURFACE ELECTRODE ITO 150

Similarly to the sample of Example 1-1, for the samples of Example 1-2and Comparative Example 1, the voltage dependence of the E.Q.E. wasmeasured. FIGS. 27 and 28 illustrate the voltage dependence of theE.Q.E. in the samples of Example 1-2 and Comparative Example 1,respectively.

As illustrated in FIG. 27, in the sample of Example 1-2, the E.Q.E. inthe infrared range increases as the electric field strength appliedacross two electrodes increases. That is, in the sample of Example 1-2,due to the increase in the absolute value of the bias voltage applied tothe lower-surface electrode, the E.Q.E. at near (near 750 nm) theabsorption peak position of ClAlPc included in the lower-sidephotoelectric conversion layer increases. In other words, modulation ofsensitivity occurred in the infrared range by switching the biasvoltage. For instance, at the wavelength corresponding to the absorptionpeak of ClAlPc, the E.Q.E. when the potential of the lower-surfaceelectrode was set to −1 V is compared with the E.Q.E. when the potentialof the lower-surface electrode was set to −5 V, the latter wasapproximately 6.55 times the former. In contrast, as illustrated in FIG.28, in the sample of Comparative Example 1, even when the bias voltageapplied to the lower-surface electrode is changed, no significant changewas observed in the graph of E.Q.E., and it is found that no modulationof sensitivity occurred in the infrared range by switching the biasvoltage.

Next, similarly to the sample of Example 1-1, for each of the samples ofExample 1-2 and the sample of Comparative Example 1, a sample havingonly the upper-side photoelectric conversion layer between thelower-surface electrode and the upper-surface electrode, and a samplehaving only the lower-side photoelectric conversion layer between thelower-surface electrode and the upper-surface electrode were produced,and the impedance of the upper-side photoelectric conversion layer andthe impedance of the lower-side photoelectric conversion layer weremeasured. The thicknesses of the upper-side photoelectric conversionlayer and the lower-side photoelectric conversion layer in the sample ofa measurement target are both 200 nm. Table 5 below lists the result ofmeasurement of impedance. Each of the values of impedance listed belowis a value when the bias voltage to the lower-surface electrode withrespect to the upper-surface electrode at −8 V and the frequency at 1Hz.

TABLE 5 DONOR- ACCEPTOR IMPEDANCE SAMPLE LAYER RATIO (Ω) EXAMPLEUPPER-SIDE DTDCTB:C₇₀ 1.2 × 10⁷ 1-2 PHOTOELECTRIC (1:9) CONVERSION LAYERLOWER-SIDE ClAlPc:C₇₀ 6.3 × 10⁴ PHOTOELECTRIC (1:1) CONVERSION LAYERCOMPAR- UPPER-SIDE DTDCTB:C₇₀ 3.0 × 10⁷ ATIVE PHOTOELECTRIC (1:4)EXAMPLE CONVERSION LAYER 1 LOWER-SIDE ClAlPc:C₇₀ 1.0 × 10⁷ PHOTOELECTRIC(1:4) CONVERSION LAYER

As seen from Table 5, in the sample of Comparative Example 1, theimpedance of the upper-side photoelectric conversion layer wasapproximately three times greater than the impedance of the lower-sidephotoelectric conversion layer, whereas in the sample of Example 1-2,the impedance of the upper-side photoelectric conversion layer wasapproximately 190 times greater than the impedance of the lower-sidephotoelectric conversion layer. Modulation of sensitivity by switchingthe bias voltage was not observed in the sample of Comparative Example1, whereas modulation of sensitivity by switching the bias voltage wasobserved in the sample of Example 1-2. This is probably because theimpedance difference between the upper-side photoelectric conversionlayer and the lower-side photoelectric conversion layers has increased.

It is to be noted that in the samples of Example 1-1 and Example 1-2,the ionization potential of DTDCTB used for formation of the upper-sidephotoelectric conversion layer is approximately 5.6 eV. The ionizationpotential of SnNc used for formation of the lower-side photoelectricconversion layer in the sample of Example 1-1 and the ionizationpotential of ClAlPc used for formation of the lower-side photoelectricconversion layer in the sample of Example 1-2 are 5.0 eV and 5.5 eV,respectively. Therefore, in the samples of Example 1-1 and Example 1-2,no potential barrier for the positive charges is formed between thelower-side photoelectric conversion layer and the upper-sidephotoelectric conversion layers. Thus, when the impedance differencebetween two photoelectric conversion layers in the multilayer structureis high to some extent, modulation of sensitivity by switching the biasvoltage is obtained even when no potential barrier for the positivecharges is present.

Example 2-1

Similarly to the sample of Example 1-1 essentially, a sample of Example2-1 was produced except that SnNc and C₇₀ were used as the material toform the upper-side photoelectric conversion layer, and rubrene and C₇₀were used as the material to form the lower-side photoelectricconversion layer. The volume ratio between SnNc and C₇₀, and the volumeratio between rubrene and C₇₀ were adjusted to be 1:4. Table 6 belowlists the material and the thickness of each layer in the sample ofExample 2-1. As illustrated in Table 6, both the thickness of theupper-side photoelectric conversion layer and the lower-sidephotoelectric conversion layer were 200 nm.

TABLE 6 THICK- LAYER MATERIAL NESS (nm) UPPER-SURFACE ELECTRODE Al 80UPPER-SIDE PHOTOELECTRIC SnNc:C₇₀ (1:4) 200 CONVERSION LAYER LOWER-SIDEPHOTOELECTRIC Rubrene:C₇₀ (1:4) 200 CONVERSION LAYER ELECTRON BLOCKINGLAYER CZBDF 10 LOWER-SURFACE ELECTRODE ITO 150

Comparative Example 2-1

Similarly to the sample of Example 2-1, a sample of Comparative Example2-1 was produced except that Rubrene and C₇₀ were used as the materialto form the upper-side photoelectric conversion layer, and SnNc and C₇₀were used as the material to form the lower-side photoelectricconversion layer. In other words, the sample of Comparative Example 2-1has a configuration in which the upper-side photoelectric conversionlayer and the lower-side photoelectric conversion layer in the sample ofExample 2-1 are swapped. Table 7 below lists the material and thethickness of each layer in the sample of Comparative Example 2-1.

TABLE 7 THICK- LAYER MATERIAL NESS (nm) UPPER-SURFACE ELECTRODE Al 80UPPER-SIDE PHOTOELECTRIC Rubrene:C₇₀ (1:4) 200 CONVERSION LAYERLOWER-SIDE PHOTOELECTRIC SnNc:C₇₀ (1:4) 200 CONVERSION LAYER ELECTRONBLOCKING LAYER CZBDF 10 LOWER-SURFACE ELECTRODE ITO 150

Similarly to the sample of Example 1-1, for the samples of Example 2-1and Comparative Example 2-1, the voltage dependence of the E.Q.E. wasmeasured. FIGS. 29 and 30 illustrate the voltage dependence of theE.Q.E. in the samples of Example 2-1 and Comparative Example 2-1,respectively.

In FIG. 29, as illustrated by a dashed ellipse S, in the sample ofExample 2-1, the E.Q.E. in the infrared range increases as the electricfield strength applied across two electrodes increases. In thisinstance, sufficient sensitivity has occurred in the infrared range atnear a point where the bias voltage applied to the lower-surfaceelectrode falls below −5V. In other words, in the sample of Example 2-1,due to the increase in the absolute value of the bias voltage applied tothe lower-surface electrode, the E.Q.E. at near the absorption peakposition of SnNc included in the lower-side photoelectric conversionlayer increases. For instance, at the wavelength (near 870 nm)corresponding to the absorption peak of SnNc, the E.Q.E. when thepotential of the lower-surface electrode was set to −3 V is comparedwith the E.Q.E. when the potential of the lower-surface electrode wasset to −10 V, the latter was approximately 4.27 times the former.

In contrast, as illustrated in FIG. 30, in the sample of ComparativeExample 2-1, both the E.Q.E. in the infrared range and the E.Q.E. in thevisible range increase as the electric field strength applied across twoelectrodes increases. That is, in the sample of Comparative Example 2-1,no distinct modulation of sensitivity occurred in the infrared range byswitching the bias voltage.

Next, similarly to the sample of Example 1-1, for each of the samples ofExample 2-1 and the sample of Comparative Example 2-1, a sample havingonly the upper-side photoelectric conversion layer between thelower-surface electrode and the upper-surface electrode, and a samplehaving only the lower-side photoelectric conversion layer between thelower-surface electrode and the upper-surface electrode were produced,and the impedance of the upper-side photoelectric conversion layer andthe impedance of the lower-side photoelectric conversion layer weremeasured. The thicknesses of the upper-side photoelectric conversionlayer and the lower-side photoelectric conversion layer in the sample ofa measurement target are both 200 nm. Table 8 below lists the result ofmeasurement of impedance.

TABLE 8 DONOR- ACCEPTOR IMPEDANCE SAMPLE LAYER RATIO (Ω) EXAMPLEUPPER-SIDE SnNc:C₇₀ 1.0 × 10⁴ 2-1 PHOTOELECTRIC (1:4) CONVERSION LAYERLOWER-SIDE Rubrene:C₇₀ 9.0 × 10³ PHOTOELECTRIC (1:4) CONVERSION LAYERCOMPAR- UPPER-SIDE Rubrene:C₇₀ 9.0 × 10³ ATIVE PHOTOELECTRIC (1:4)EXAMPLE CONVERSION LAYER 2-1 LOWER-SIDE SnNc:C₇₀ 1.0 × 10⁴ PHOTOELECTRIC(1:4) CONVERSION LAYER

As seen in Table 8, in the sample of Comparative Example 2-1, theimpedance of the upper-side photoelectric conversion layer is smallerthan the impedance of the lower-side photoelectric conversion layer. Incontrast, in the sample of Example 2-1, the impedance of the upper-sidephotoelectric conversion layer is greater than the impedance of thelower-side photoelectric conversion layer. However, the ratio of theimpedance of the upper-side photoelectric conversion layer to thelower-side photoelectric conversion layer is approximately 1.1 times,and a large difference was observed between the lower-side photoelectricconversion layer and the upper-side photoelectric conversion layer.

Here, when attention is focused on the ionization potentials of rubreneand SnNc, the ionization potential of rubrene is 5.35 eV and theionization potential of SnNc is 5.0 eV. Therefore, in the sample ofExample 2-1, for the positive charges that move toward the lower-surfaceelectrode, a potential barrier of 0.35 eV is present between the HOMOlevel of rubrene and the HOMO level of SnNc (see FIG. 21). In contrast,in the sample of Comparative Example 2-1, for the positive charges thatmove toward the lower-surface electrode, no potential barrier is presentbetween the HOMO level of rubrene and the HOMO level of SnNc. It ispresumed that the reason why no distinct modulation of sensitivity inthe infrared range was observed in the sample of Comparative Example2-1, and yet distinct modulation of sensitivity in the infrared rangewas observed in the sample of Example 2-1 is that a potential barrierfor the positive holes was formed between two photoelectric conversionlayers.

Example 2-2

The materials listed in Table 9 below were deposited sequentially on aglass substrate by vacuum deposition, and thus a sample of Example 2-2was produced. In the formation of the lower-side photoelectricconversion layer, ClAlPc and C₆₀ were co-evaporated, and in theformation of the upper-side photoelectric conversion layer, α-6T and C₇₀were co-evaporated. In the formation of the lower-side photoelectricconversion layer, the conditions for vapor deposition were adjusted sothat the volume ratio between ClAlPc and C₆₀ becomes 1:4, and in theformation of the upper-side photoelectric conversion layer, theconditions for vapor deposition were adjusted so that the volume ratiobetween α-6T and C₇₀ becomes 1:1.

TABLE 9 THICK- LAYER MATERIAL NESS (nm) UPPER-SURFACE ELECTRODE Al 80UPPER-SIDE PHOTOELECTRIC α-6T:C₇₀ (1:1) 60 CONVERSION LAYER LOWER-SIDEPHOTOELECTRIC ClAlPc:C₆₀ (1:4) 60 CONVERSION LAYER ELECTRON BLOCKINGLAYER CZBDF 10 LOWER-SURFACE ELECTRODE ITO 150

FIG. 31 illustrates an energy diagram for the sample of Example 2-2. Asillustrated in FIG. 31, the ionization potentials of ClAlPc and α-6T are5.5 eV and 5.3 eV, respectively, and in the sample of Example 2-2, apotential barrier of 0.2 eV is formed between the HOMO level of ClAlPcand the HOMO level of α-6T.

Similarly to the sample of Example 1-1, for the sample of Example 2-2,the voltage dependence of the E.Q.E. was measured. FIG. 32 illustratesthe voltage dependence of the E.Q.E. in the sample of Example 2-2. Asillustrated in FIG. 32, in the sample of Example 2-2, due to theincrease in the absolute value of the bias voltage applied to thelower-surface electrode, the E.Q.E. at near (near 440 nm) the absorptionpeak position of α-6T increases. In other words, the E.Q.E. in thevisible range increases. That is, in this instance, the effect ofmodulation of sensitivity by switching the bias voltage in the visiblerange is obtained.

Comparative Example 2-2

Similarly to the sample of Example 2-2, a sample of Comparative Example2-2 was produced except that the material to form the upper-sidephotoelectric conversion layer and the material to form the lower-sidephotoelectric conversion layer are swapped. Table 10 below lists thematerial and the thickness of each layer in the sample of Example 2-2.

TABLE 10 THICK- LAYER MATERIAL NESS (nm) UPPER-SURFACE ELECTRODE Al 80UPPER-SIDE PHOTOELECTRIC ClAlPc:C₆₀ (1:4) 60 CONVERSION LAYER LOWER-SIDEPHOTOELECTRIC α-6T:C₇₀ (1:1) 60 CONVERSION LAYER ELECTRON BLOCKING LAYERCZBDF 10 LOWER-SURFACE ELECTRODE ITO 150

FIG. 33 illustrates an energy diagram for the sample of ComparativeExample 2-2. As seen from FIG. 33, in this instance, no potentialbarrier for the positive charges is formed between the HOMO level ofClAlPc and the HOMO level of α-6T.

Similarly to the sample of Example 1-1, for the sample of ComparativeExample 2-2, the voltage dependence of the E.Q.E. was measured. FIG. 34illustrates the voltage dependence of E.Q.E. in a sample of ComparativeExample 2-2. As illustrated in FIG. 34, in the sample of ComparativeExample 2-2, even when the bias voltage applied to the lower-surfaceelectrode is changed, no significant change was observed in the graph ofE.Q.E., and no modulation of sensitivity occurred by switching the biasvoltage.

It is found from FIGS. 29 to 34 that sensitivity modulation can beachieved through switching the bias voltage by forming a potentialbarrier for the positive charges between the HOMO level of the materialof which the upper-side photoelectric conversion layer is composed andthe HOMO level of the material of which the lower-side photoelectricconversion layer is composed. From comparison between Example 2-2 andComparative Example 2-2, distinct increase in the E.Q.E. can be achievedeven in the visible range by appropriately selecting materials for thetwo photoelectric conversion layers in the multilayer structure.

It is found from comparison between Example 2-2 and Comparative Example2-2 that when the material for one photoelectric conversion layer has anionization potential greater than the ionization potential of thematerial for the other photoelectric conversion layer by 0.2 eV or more,the effect of distinct increase in the E.Q.E. can be achieved in aspecific wavelength range in addition to the infrared range, the onephotoelectric conversion layer being one of two photoelectric conversionlayers included in the multilayer structure in the photoelectricconversion structure and being closer to an electrode relatively low inpotential (the lower-surface electrode in this instance). For instance,the ionization potential of Si(OSiR₃)₂Nc and the ionization potential ofCuPc are 5.4 eV and 5.2 eV, respectively, and thus when Si(OSiR₃)₂Nc andCuPc are used as the first material Included in the first photoelectricconversion layer 64 a and the second material Included in the secondphotoelectric conversion layer 64 b, respectively, it is expected thatdistinct modulation of sensitivity in the visible range occurs. Insteadof rubrene of Example 2-2, CuPc may be used.

As already described, the photoelectric conversion structure 66 of theimaging device 100F described with reference to FIG. 36 may be composedof a photoelectric conversion material including at least two types ofmaterials that are the first material serving as a donor and the secondmaterial serving as an acceptor. Therefore, it is possible to use thefirst photoelectric conversion layer 64 a or the second photoelectricconversion layer 64 b as the photoelectric conversion structure 66.Alternatively, as in the mixed layer in the sample of theabove-described Reference Example 1, a layer formed by co-evaporatingthree materials of SnNc, DTDCTB, and C₇₀, for instance, can be used asthe photoelectric conversion structure 66.

With use of the photoelectric conversion structure 66 having such aconfiguration, the sensitivity in the imaging cell 20 can beelectrically changed by changing the potential difference applied acrossthe electrodes by which the photoelectric conversion structure 66 isinterposed. Therefore, for instance, two types of image signals forsynthesizing a high dynamic range can be collectively obtained by makingthe potential difference Φx between the opposite electrode 62 x and thepixel electrode 61 x different from the potential difference Φy betweenthe opposite electrode 62 y and the pixel electrode 61 y.

(Typical Example of Photoelectric Current Characteristic inPhotoelectric Conversion Layer)

Furthermore, a photoelectric conversion structure that exhibitsphotocurrent characteristic as described below is used for thephotoelectric converters PC or PC2, and the potential difference Φbetween the pixel electrode 61 and the opposite electrode 62 is reducedto some extent, thereby making it possible to suppress movement ofsignal charges already accumulated in the charge accumulation region tothe opposite electrode 62 via the photoelectric conversion structure 64or the photoelectric conversion structure 66, and further accumulationof the signal charges in the charge accumulation region after thepotential difference is reduced. Consequently, the function of a shuttercan be electrically achieved by controlling the magnitude of the biasvoltage to be applied to the photoelectric conversion structure.Therefore, for instance, a global shutter function can be achievedwithout separately providing a device such as a transfer transistor ineach of the multiple imaging cells.

FIG. 35 illustrates a typical photocurrent characteristic of aphotoelectric conversion structure according to Embodiments of thepresent disclosure. The thick solid graph in FIG. 35 illustrates anexemplary I-V characteristic of the photoelectric conversion structureunder irradiation with light. It is to be noted that FIG. 35 alsoillustrates an instance of I-V characteristic under no irradiation withlight by a thick dashed line.

FIG. 35 illustrates the change in the current density between the majorsurfaces of the photoelectric conversion structure (e.g., thephotoelectric conversion structure 64, 64A, 64B, or 66) when the biasvoltage to be applied across the two major surfaces is changed undercertain illumination. In the present description, the forward directionand the reverse direction in the bias voltage are defined as follows:When the photoelectric conversion structure has a structure of junctionbetween a layered p-type semiconductor and a layered n-typesemiconductor, a bias voltage that causes the potential of a layer ofp-type semiconductor to be higher than the potential of a layer ofn-type semiconductor is defined as the bias voltage in the forwarddirection. On the other hand, a bias voltage that causes the potentialof a layer of p-type semiconductor to be lower than the potential of alayer of n-type semiconductor is defined as the bias voltage in thereverse direction. Similarly to the case where an inorganicsemiconductor material is used, in the case where an organicsemiconductor material is used, the forward direction and the reversedirection can be defined. When the photoelectric conversion structurehas a bulk heterojunction structure, as schematically illustrated inFIG. 1 in Japanese Unexamined Patent Application Publication No. 5553727mentioned above, a p-type semiconductor appears more often than ann-type semiconductor on one of the two major surfaces of a bulkheterojunction structure, which face respective electrodes, and ann-type semiconductor appears more often than a p-type semiconductor onthe other major surface. Therefore, a bias voltage that causes thepotential of one major surface in which a p-type semiconductor appearsmore often than an n-type semiconductor to be higher than the potentialof the other major surface in which an n-type semiconductor appears moreoften than a p-type semiconductor can be defined as the bias voltage inthe forward direction.

As illustrated in FIG. 35, for instance, the photoelectric currentcharacteristic of the photoelectric conversion structure 64A isschematically characterized by first to third voltage ranges. The firstvoltage range is a reverse bias voltage range in which the absolutevalue of output current density increases as a reverse direction biasvoltage increases. The first voltage range may be a voltage range suchthat a photoelectric current increases as the bias voltage appliedacross the major surfaces of the photoelectric conversion structureincreases. The second voltage range is a forward bias voltage range inwhich the absolute value of output current density increases as aforward direction bias voltage increases. In other words, the secondvoltage range may be a voltage range such that a forward directioncurrent increases as the bias voltage applied across the major surfacesof the photoelectric conversion structure increases. The third voltagerange is a voltage range between the first voltage range and the secondvoltage range.

The first to third voltage ranges can be distinguished by the slope of agraph of photoelectric current characteristic when linear vertical axisand horizontal axis are used. For reference, in FIG. 35, an averageslope of the graph in each of the first voltage range and the secondvoltage range is indicated by a dashed line L1 and a dashed line L2,respectively. As illustrated in FIG. 35, a rate of change in the outputcurrent density for an increase in the bias voltage in the first voltagerange, the second voltage range, and the third voltage range isdifferent from range to range. The third voltage range is defined as thevoltage range in which the rate of change in the output current densityfor the bias voltage is lower than the rate of change in the firstvoltage range and the rate of change in the second voltage range.Alternatively, the third voltage range may be determined based on arising or falling position in the graph illustrating the I-Vcharacteristic. The third voltage range is typically a voltage rangegreater than −1 V and smaller than 1 V. In the third voltage range, evenwhen the bias voltage is changed, the current density between the majorsurfaces of the photoelectric conversion structure hardly changes. Asillustrated in FIG. 35, in the third voltage range, the absolute valueof current density is typically 100 μA/cm² or less.

For instance, the potential of the pixel electrode 61 is adjusted byswitching the voltage applied to the first voltage line 31 from thevoltage supply circuit 41, and thereby the potential difference betweenthe pixel electrode 61 and the opposite electrode 62, in other words,the bias voltage applied across the major surfaces of the photoelectricconversion structure at the start of a signal accumulation period canfall within the third voltage range. A state can be achieved in whichsubstantially no charges are moved between the photoelectric conversionstructure and the electrodes, by maintaining the bias voltage appliedacross the major surfaces of the photoelectric conversion structurewithin the third voltage range. In short, an electrical shutter can beachieved.

The imaging device in the present disclosure is applicable to, forinstance, an image sensor and particularly, for photographing an objectmoving at a high speed. The imaging device in the present disclosure canbe used for a camera for machine vision represented by a digital camera,a camera for medical use, and a camera for robots. A camera for machinevision may be utilized for input for performing, for instance,determination, classification of a state of products or detection of adefective in a production plant by using image recognition. Acquisitionof an image utilizing the infrared light is also possible byappropriately selecting the material for the photoelectric conversionstructure, and the voltage applied to the first signal line. Thus, anembodiment in the present disclosure is also useful for a securitycamera, and a camera mounted and used in a vehicle. The in-vehiclecamera may be utilized for input to a control device for running thevehicle safely, for instance. Alternatively, the in-vehicle camera maybe utilized for assisting an operator for running the vehicle safely. Aninfrared image can be utilized for sensing, such as distance detectionand object recognition.

What is claimed is:
 1. An imaging device comprising: a first imagingcell including a first photoelectric converter including a firstelectrode, a second electrode, and a first photoelectric conversionlayer between the first electrode and the second electrode, and a firstreset transistor one of a source and a drain of which is coupled to thefirst electrode; a second imaging cell including a second photoelectricconverter including a third electrode, a fourth electrode, and a secondphotoelectric conversion layer between the third electrode and thefourth electrode, and a second reset transistor one of a source and adrain of which is coupled to the third electrode; first voltage supplycircuitry configured to supply a first voltage to the other of thesource and the drain of the first reset transistor; and second voltagesupply circuitry configured to supply a second voltage different fromthe first voltage to the other of the source and the drain of the secondreset transistor.
 2. The imaging device according to claim 1, furthercomprising: a first inverting amplifier having a first inverting inputterminal, a first non-inverting input terminal, and a first outputterminal; and a second inverting amplifier having a second invertinginput terminal, a second non-inverting input terminal, and a secondoutput terminal, wherein the first imaging cell includes a first signaldetection transistor having a gate coupled to the first electrode, oneof a source and a drain of the first signal detection transistor beingcoupled to the first inverting input terminal, the second imaging cellincludes a second signal detection transistor having a gate coupled tothe third electrode, one of a source and a drain of the second signaldetection transistor being coupled to the second inverting inputterminal, the other of the source and the drain of the first resettransistor is coupled to the first output terminal, the other of thesource and the drain of the second reset transistor is coupled to thesecond output terminal, the first voltage supply circuitry is coupled tothe first non-inverting input terminal, and the second voltage supplycircuitry is coupled to the second non-inverting input terminal.
 3. Theimaging device according to claim 1, wherein each of the firstphotoelectric conversion layer and the second photoelectric conversionlayer includes a first layer and a second layer stacked one on theother, and impedance of the first layer is greater than impedance of thesecond layer.
 4. The imaging device according to claim 1, wherein eachof the first photoelectric conversion layer and the second photoelectricconversion layer includes a first layer and a second layer stacked oneon the other, the first layer includes a first material, the secondlayer includes a second material, and an ionization potential of thefirst material is greater than an ionization potential of the secondmaterial by 0.2 eV or more.
 5. The imaging device according to claim 3,wherein the first layer includes a first material, the second layerincludes a second material, and the first material and the secondmaterial are both electron-donating molecules.
 6. The imaging deviceaccording to claim 4, wherein the first material and the second materialare both electron-donating molecules.
 7. An imaging device comprising: afirst imaging cell including a first photoelectric converter including afirst electrode, a second electrode, and a first photoelectricconversion layer between the first electrode and the second electrode,and a first capacitor having one end coupled to the first electrode; asecond imaging cell including a second photoelectric converter includinga third electrode, a fourth electrode, and a second photoelectricconversion layer between the third electrode and the fourth electrode;and voltage supply circuitry configured to selectively supply a firstvoltage and a second voltage different from the first voltage to theother end of the first capacitor.
 8. The imaging device according toclaim 7, wherein the second imaging cell includes a second capacitorhaving one end coupled to the third electrode, a capacitance value ofthe second capacitor being different from a capacitance value of thefirst capacitor, and the voltage supply circuitry is coupled to theother end of the second capacitor.
 9. The imaging device according toclaim 8, wherein the voltage supply circuit selectively supplies thefirst voltage and the second voltage to both of the other end of thefirst capacitor and the other end of the second capacitor.
 10. Theimaging device according to claim 7, wherein each of the firstphotoelectric conversion layer and the second photoelectric conversionlayer includes a first layer and a second layer stacked one on theother, and impedance of the first layer is greater than impedance of thesecond layer.
 11. The imaging device according to claim 7, wherein eachof the first photoelectric conversion layer and the second photoelectricconversion layer includes a first layer and a second layer stacked oneon the other, the first layer includes a first material, the secondlayer includes a second material, and an ionization potential of thefirst material is greater than an ionization potential of the secondmaterial by 0.2 eV or more.
 12. The imaging device according to claim10, wherein the first layer includes a first material, the second layerincludes a second material, and the first material and the secondmaterial are both electron-donating molecules.
 13. The imaging deviceaccording to claim 11, wherein the first material and the secondmaterial are both electron-donating molecules.
 14. An imaging devicecomprising: a first imaging cell including a first photoelectricconverter including a first electrode, a second electrode, and a firstphotoelectric conversion layer between the first electrode and thesecond electrode, and a first signal detection transistor having a gatecoupled to the first electrode; a second imaging cell including a secondphotoelectric converter including a third electrode, a fourth electrode,and a second photoelectric conversion layer between the third electrodeand the fourth electrode, and a second signal detection transistorhaving a gate coupled to the third electrode; first voltage supplycircuitry configured to supply a first voltage to the second electrode;and second voltage supply circuitry configured to supply a secondvoltage different from the first voltage to the fourth electrode. 15.The imaging device according to claim 14, wherein each of the firstphotoelectric conversion layer and the second photoelectric conversionlayer includes a first layer and a second layer stacked one on theother, and impedance of the first layer is greater than impedance of thesecond layer.
 16. The imaging device according to claim 14, wherein eachof the first photoelectric conversion layer and the second photoelectricconversion layer includes a first layer and a second layer stacked oneon the other, the first layer includes a first material, the secondlayer includes a second material, and an ionization potential of thefirst material is greater than an ionization potential of the secondmaterial by 0.2 eV or more.
 17. The imaging device according to claim15, wherein the first layer includes a first material, the second layerincludes a second material, and the first material and the secondmaterial are both electron-donating molecules.
 18. The imaging deviceaccording to claim 16, wherein the first material and the secondmaterial are both electron-donating molecules.